I am hearing the siren call of a mirrorless camera...

Backstory, I have a Nikon J1 that I used for years and I loved it. It was hard to take a bad picture on that thing, and it was small and light and easy to use. The only limitation was zoom lenses, it was limited to 120mm unless you wanted to drop big money. So I move on to a Nikon D5100, and have added a few lenses over the years including a 300mm lens that does what I want. (shooting at the autocross for fun) It has been a reliable workhorse, but the FPS is pretty low, the focus is not as fast or sharp as I'd like, and it struggles with light settings with the zoom in place.

When looking at the next step, something smaller, lighter, and easier like my old J1 makes me happy. Hey, the writing is already on the wall, all the big players are discontinuing their entry-level DSLRs. So a mirrorless camera is the way to go, but my budget is "I really need that $$$ for other things". So I am thinking about finding a used camera that includes a longer lens, even if its not from Nikon. Should I run away from Pentax or Olympus? Should I sell my body so I can afford what I really want? Is there some secret used-camera store with crazy bargains? 

I'm just here to mute this thread so it doesn't end up costing me money, but I wish all of you the best of luck.

Edit. Mine is the A7ii. 

I picked up a Sony A7ii 3-4 years ago on sale and have collected lenses from 25mm through a 150-600 mm. 

I love the camera but don't spend as much time with it as I'd like to. 

If I was shopping now, I'd do the same thing but I'd probably spend the extra to get the A7Riii with the higher resolution. I like being able to blow pictures up without them getting grainy. 

I do all of my used shopping at KEH. In fact, I placed an order with them on Friday. 

I shoot in the canon ecosystem, and of their current crop, the R10 is a spectacular fast-action outdoor camera. Able to shoot at 20+ FPS in e-shutter or 15 FPS with a full mechanical shutter with full AF/AE, and you get easy access to all the legacy EF mount lenses with a cheap adapter. I got mine refurbished for $600, and if you're patient you should be able to get the same deal from Canon USA. Price history here, and you can set up an e-mail alert for sales:
Canon EOS R10 with 18-45mm Lens | Canon Camera and Lens Deals - Canon Price Watch (cpricewatch.com)

The RF 18-45 is one of the worst lenses in the canon line-up, but it's still decent, and it's VERY small. I use mine a fair bit despite having other "better" lenses when I just want a walk-around camera.

Add in the RF 100-400 when it's on a refurb sale and you've got the best budget wildlife setup you can get right now, and it'll be great for autocross.

I also shoot with a Canon EOS R, which is great for portraits but not great for fast action and it's outdated, don't bother with it. The R10 for action/wildlife, or the R8 as a more expensive fast-action / video / portrait body is great, just remember that they use different size sensors.

If you're looking used, the Sony A6400 or A6600 are both cheap, small, and can take a ton of lenses, but I never loved how their bodies felt in hand.

Don't bother with the Nikon mirrorless unless you're an avid wildlife shooter with cash to burn, that's their current niche. I wouldn't bother with Olympus or Panasonic at this point.

The Fuji options are generally great and well rounded as well, but I think that the best value if you're starting from scratch is the Canon R10.

If it helps, I actually prefer the weight of a 7D MkII when using the big lenses. My go-to for action: 7D Mk with a battery grip and a 70-200 f/2.8. The wight of the camera seems to nicely balance the weight of the lens.

I have also (happily) purchased refurb gear from Canon and Fuji. 

I switched to mirrorless and ended up using my camera less.  Possibly the less intuitive interface and the learning curve.  Honestly, although smaller, it's not enough smaller to make a difference.  I'm either lugging a dedicated camera with me, or I'm using my phone.  And If I'm bringing one with me... turns out the size doesn't make that big of a difference.  I have a Sony A6400.  I need to check to make sure it works, but if you want a cheap body, let me know.  (I broke the lens that came with it).

I have a few different Nikon APS-C (I forget the terminology) lenses from an old D60, I'm thinking about just grabbing a D3000+ body off FB Marketplace and just enjoying it again.

And I did make the jump to mirrorless with a Fujifilm X-T3 and X-Pro3. I mostly use those for my own stuff. 

Siren call, eh?  Does it sound anything like this?

I've got a nice collection of Canon L glass EF series but haven't been shooting much with my 80D for a while now.  Nice to know that an adapter exists should I decide to wade back in with a mirrorless body.  

over the last 6 years, I've shot on a Canon 6D, a Canon 5D MkIII, a Sony A7ii, a Sony a6000, and a Sony a6400 (I've owned all but the 5D MkIII).

I'm real comfortable with the Canon system, and especially the way the "zoom" moves (Nikon, Sony, and a bunch of others zoom with the opposite rotation {Sigma zooms the same as Canon}).  So, I frequently miss shots because I'm zooming out instead of in, or in instead of out.  Dunno if this is a "thing" for you... but if so, I'd suggest sticking with a Nikon mirrorless or a Sony mirrorless.

The next question is:  do you primarily use the screen for focus/composition... or is have an "eye" viewfinder important?  have you shot with the viewfinder not centered on the lens?  I've further missed shots with the a6000 and a6400 with the Sony 70-300mm zoom, because I couldn't go from looking at the subject and raising the camera to my eye.  This might be a "me" thing, though... because I shoot "crossed-up" (i.e. I'm right hand and eye dominant, but I shoot with my left eye looking through the viewfinder.  so, ymmv... but something to think about.

if you are used to a screen-based use... then a Nikon Z30 is probably a good buy.  Otherwise if having a viewfinder is important, the find a Z50.

I wouldn't suggest a Sony, because the colors are probably going to "feel off", if you're used to Nikon (I feel that with the Sony compared to my Canons)... and because the APSC's are all "rangefinder"-ish style with the viewfinder offset to the left side.

If you can make the viewfinder work and are ok with the colors, though, then an used a6100, a6300, a6400 would be a good avenue to persue.

another possibility is the Micro4/3rds system (older Olympus now "OM Systems" and Panasonic), which have sensors 1/2 the size of 35mm film... which means the lenses can be real portable... but the sensor is still quite a bit larger than the one in your J1.

this is a good channel to get a handle on the options in Micro4/3rds... especially used gems:

https://www.youtube.com/@MicroFourNerds

this is a good "overview" of sorts

or you can pick your way through this:

Nikon's mirrorless cameras are fine, and with a Z-mount adapter, depending on the age of the lenses you already own, they'll work on the camera.

In particular, if you want small (crop sensor), the Z50 is a decent, inexpensive option. A semi-pro I know and trust uses it for travel and loves it. For used, the Z6 is also an inexpensive option. The Z6ii is better, and full frame, and is expected to be replaced by a Z6iii very soon, which will drive prices down. Caveat for these is, their focus tracking can be challenging for fast moving sports and animals. The newer Z9 and Z8 are very much optimized for sports, birds in flight, and other fast stuff, and their $$$ price tags reflect this. The imminent Z6iii will almost certainly inherit most of those cameras' focus-tracking tech. 

I'll second David's https://www.KEH.com recommendation. They won't be the cheapest source for used gear, but they have a good reputation and generally rate gear condition conservatively. Another option is Nikon itself, which sells refurbished gear on their site. If memory serves, you can sometimes buy Nikon refurbs from B&H and Adorama, too. I personally wouldn't buy used from these retailers -- I like both but have read too many bad stories about issues with used gear. 

I have an Olympus OM because they work with legacy glass really well 

The smaller sensor mean more of the sensor is in the sweet spot of an old lens like my Minolta stuff.

Makes a good lens into a great lens.

Thank you for all of the advice, much appreciated. I fell down a deep rabbit hole of research and comparisons, but in the end, the decision was pretty straightforward. My budget was $1K, and for that, I wanted a body, prime lens, and telephoto lens. That limited me to new camera bundles, as I could not buy all three on the used market for less. So my choices were narrowed to the Nikon Z30 or the Canon R50 or the Sony Alpha 6100. 

I am familiar with Nikon, and have a few lenses that I could use with an adapter, but I really wanted to take advantage of the smaller size of the APS-C format so that wasn't a big deciding factor. Of the three, only one offered a viewfinder: the R50. I never even considered that before this thread, but it was a deal-breaker for me. The R50 also has a built-in flash and seemed to perform better in action shooting - it even has a race car focus tracking feature - and ranked higher overall in most comparisons. 

For my specific use case, I think any of the options would have been just fine, but the R50 seemed to be just a little more fine than the rest. 
 

An update on ordering: If you see the price you want from B&H, just buy it. I have never had a bad experience with them. Unfortunately, I did not follow my own advice and bought from WalMart.com because they had a good price on a bundle that included a bunch of extra stuff. Most of it was pretty cheap junk, but a few memory cards and filters made it worthwhile. The order arrived in time for my next event, but was missing the 55 -210 lens. Many calls and emails were exchanged, and Wal Mart basically said that I was actually dealing with a reseller and there was no way that they were going to get me the lens, so just return the whole thing. UGH. Hopefully my money will be refunded before my son goes to college (6 years from now). 

While this was all going down, I was in my local Best Buy and they had one of the same bundles on the shelf at a great price. I bought it because I could open the box and confirm all contents before exchanging money. Now I have the camera and lenses that I wanted. 

I was able to take some shots with the first camera as a test, and so far I am very happy about the lighter weight and simplicity of it. 

The odds are high that Walmart will refund you the money, forget to pick up the first camera, and send you another one. I don't think the right hand and the left hand don't share a brain. 

I ended up with an extra microwave and a set of tires due to their piss-poor inter-office communications. Trying to explain it to them is impossible and a waste of time.

Sweeeeet! Enjoy the new gear!