I can't see your ad from work, but I've owned a couple of '86 XR4Tis, so I'll throw my two cents in.
My first one was purchased with a disassembled automatic transmission in the hatch/trunk.
I didn't bother putting an auto back in. I sourced a used T9 (the stock 5 speed), driveshaft, speedo cable, pedal set, flywheel, clutch, etc, and bolted it all together.
I never did anything as far as upgrades on either car. They were fun daily drivers. Mostly fun because 99+% of the general population has no idea what they are.
You have to keep good plug wires on them. I just got parts store ones and waranteed them when it started cutting out under boost.
PCV valves are a consumable maintenance item (like spark plug wires).
Suspension parts are not easy to come by (think bushings and such). They were out there, but not cheap enough for me.
In my mind, the real problem with these cars is that they were as much or more "luxury" as they were "sport" when new. You'll have a car full of non-working accessories. If this bothers you, steer clear of them in general. If you're ok with disabling, bypassing, deleting, or dealing with quirky electronics and options that don't work anymore, you'll do fine.
I'd say it's not a good car for the anal retentive.
In stock form, the car shines in "passing" situations. I was never happy with "off the line" performance, but if you want to go from 50 mph to 80 mph RIGHT NOW...they'll do it.
I'll probably own another again someday, but I'm not really looking out for one. When one comes along cheap, I'll pick one up. (well I say that...and now I have the urge to search for XR4Tis within 150 miles of me ;)).
Clem