My knowledge is limited to the M20s, but for any car that age check for rust (floors, fenders, rockers, rear license plate, A-pillars are common areas), leaks (oil, coolant), uneven tire wear, any funky electronic behaviour. This is probably not the best place to find info on e30s. I suggest you check out r3vlimited.com, there's lots and lots of info there that you can search for.
Also;
http://www.maxbimmer.com/forums/showthread.php?t=32438