I'm biased but I think they are great cars.
Some flaws, tho.
As noted earlier, 85 was the first year of the 16v engine - the head is 1-year-only. Not really a big deal unless you were to need a head.
But the wiring harnesses in 85 and some 86 Turbos were *terrible*: total insulation failure. If the car hasn't had a whole mess of underhood wiring replaced, it could be a PITA - either you replace one wire at a time or you find an intact '86T or '87T harness to graft in.
On 86+, modifying the boost controller, or APC, is really simple (a couple resistors and twist some knobs) but the 85 is different and I'm not sure it can be modified similarly. If you do increase boost, say, with a check valve, you can get a little more fuel in open loop by installing a 3 bar fuel reg from a nonturbo (Turbo is 2.5 bar). If you get too greedy with boost, on an 85, a pressure sensor will suddenly kill fuel. It's not subtle.
The car has double wishbones up front and you can easily get more camber with shims, which I highly recommend.
If memory serves, the 85T lacks a rear sway bar. I prefer a car with both. Get one from a later Turbo (grab all fasteners, too). I can't recall when NAs got bars.
If you want to raise the car, pieces of cutting board (uhmw) are what I've used in the past. In the front, the thickness of the cutting board will *double* the increase in ride height; in the rear its more 1:1. Fair warning: removing the front springs is tough.
Transmissions evolved over the years, so while there are some parts that'll fit in any of them, there are others that won't. I have a good local source for trans parts but he hoards and is creative with repurposing stuff.
Yes, pinion bearings are the usual failure. If you rebuild soon after they start to whine, you will have avoided needing a new one.
Speaking of evolution, other stuff evolved, too (fuel system, for example) so if you get the car and need stuff from the junkyard, do your homework to make sure the car has what you need.
The difficulty finding 5th gear noted above is due to the shifter going out of adjustment - a pin at the bottom that controls movement to 5th fails to catch a plastic gate.
If you have mechanical sympathy, the transmissions will thank you.
E-brake is on the front on an 85.these brakes are a bit of a pain and often clunk when applied going backwards. You can't upgrade to later better more normal stuff (e-brake in rear) without swapping spindles, rear axle, and wheels.
My buddy and I did Alcan 5000 in an 85 T with 250k+ miles on it. We did other TSDs for a bunch of years. Car was pretty reliable but we worked hard to make sure it was in good shape. I own an '86 that I converted to turbo and an 89 900T. Lmk if you have any questions. Oh, also, Mezzanine here is a longtime 900 owner.
Sorry this is disjointed - I can't cut and paste on my phone...