Here are some common problems with the E30:
On the M20 and M40 engines, insure timing belts are changed every 60,000 miles. Change the water pump at the same time. Adjust valves with every tuneup.
Head bolts should never be reused if the head is removed. Six cylinder oil filter housings can leak, but it's not that common. Always use BMW spec oil filters. Watch for rust just behind the front shock towers, it's easily fixable if caught early and often minor. Rust can form around the rear number plate lights. Water can seep into trunk. Overheating in traffic may be a sign of a broken cooling fan clutch. Rattles from the rear may be worn rear shock absorber mounts. A wobbly front or vibration when braking end could well be worn control arm bushings. Reversing light switch mechanism can fail. Lack of power on Motronic cars is often due to a worn rotor arm. Fuel tank eventually rusts where the flexible filler pipe joins it.
Tips from John G. Burns
http://www.unixnerd.demon.co.uk/e30.html
and BMW World