Lots of reading to get to this point.. but none of my opinions changed
First off, if you already have a volvo wagon and you think the PT is too small, DO NOT get a mazda5. Its space advantages over what you already have are dubious AT BEST. Id also take the Rondo right off the list for pretty much the same reason, to a slightly lesser extent. I think that MPVs could still be considered but also borderline. You need a FULL SIZE MINIVAN!! IMO the mazda5, mpv, and rondo are NOT even close to the same size category as a Grand Caravan.
You want the Grand Caravan for many reasons: It is cheap It is HUGE on the inside It is really really reliable (as long as the PO didnt abuse the tranny). It is fairly cheap and easy to work on (including rear plugs AND rebuilding the trans, in the grand scheme of things..) It is extremely versatile on the inside. It can carry a roof cargo box, a hitch tray, and even tow small things (3000lbs is my record with a grand caravan).
Get one with 2 benches. They are interchangeable and thats a big bonus. Having the 3-across seat in the middle gives you crazy legroom and room for 3 across with a HUGE cargo area in the back. Skip on the captains chairs. Not only do you lose a seating position but rear-seat access becomes more annoying imo. Get one with a roof rack or a hitch or both. There is some cause to be concerned if the PO towed with it, but there's no reason YOU shouldnt install a hitch yourself, because then you can use hitch trays and pull broken cars around your yard with the hitch's safety chain loops. It would also handle a popup camper or a small cargo or utility trailer just fine! The rear plugs on the 3.3/3.8s are only hard if you have fat arms, no wobble tools, and cant take a little pain! If you really dont want to deal with the slight annoyance, get a 3.0 model. The rear plug access on those is much easier and power is still adequate (and more easily improved if you're into modding your minivan).
The main thing i would look at with a used caravan is a TRANSMISSION COOLER. If it doesnt have one, i would be worried. Any fluid changes on top of that are nice but really it takes a LOT of fluid changes to equal the benefit of just having a trans cooler. I think i changed the fluid in my parents' 97 GC twice.. it made it to 204k (with a cooler from day one) before the seals in the trans got hard and it started acting up when cold. I rebuilt it myself, replaced no hard parts, and it was still kicking just fine at 240k when they traded it in on cash for clunkers. As a person who has rebuilt transmissions for a living at one point, i really like the 604 trans in the vans, but you HAVE to have a cooler on it. PERIOD! I think overall its a better design than the 700/4l60 you get in the Astro, which you can treat like a princess and it will STILL break the sun shell on you for no damn reason and pop up with other weird issues like the boost valve that you never run into on a 604.