I bought mine (96 base model 318ti) a year ago with 120k on the clock. It has been a very fun and practical car. Great gas mileage and awesome cargo capacity with the rear seats folded down.
Sport models got upgraded suspension. Different springs and sway bars. Front suspension is the same as any non-M e36 and the rear is the same as the z3/e30. The only year with a factory limited slip is 95. The club sport editions and the winter package cars got them. Limited slips from e30's are basically a bolt in job.
Check for rust around the rubber hockey-puck jacking points on the side of the car. That's where I saw rust on the other one I looked at.
The other issue on the M44 models (96+) is the plastic cooling system fittings. There are 3 that are prone to cracking and should be replace every ~100k miles. They are cheap, but a bit of a pain to replace. I did a complete cooling system overhaul including new hoses, radiator, plastic pieces, water pump, etc for about $500 in parts. The bottom ends are pretty much bullet proof. I have heard of issues with the cam gears/timing chain coming apart and destroying the engine. That was on an engine with 240k miles.
Properly bleeding the cooling system is essential to getting reliable engine operation. Not getting all of the air out can result in over heating and blown head gaskets.
Like gimp said, 318ti.org is a great source of information. Ignore the guys who wish they'd bought a VW over there and you'll be okay :)