the cooling system is the main plague of the e36, make sure it is replaced or at least has a new water pump as stated above
all bushings should be inspected or replaced if worn or cracked (check out the guibo, which is on the driveshaft of the car, and u must check out the rear shock mounts)
the oil pump nut is notorious for backing off the oil pump (during spirited driving), thus starving ur engine's head of oil and destroying valves and cams. make sure that sucker is welded in place or tied down with metal wire
check the suspension towers for cracks or mushrooming, if the factory suspension is still in there (which only lasts 60k miles) i would be wary of buying the car.
cooling system again, lol
other than that, they are pretty solid cars. excellent handling. great looking, and that motor is a beast. with a thicker head gasket and a turbo kit, ppl are seeing 550whp on stock internals, pretty crazy right? also this car has a HUGE following behind it, so dont worry about support when u need it
check out bimmerforums.com (been a member for 4 years) for help when u need it. ppl will actually come to ur house and give u a hand with ur car for a beer. lol. and there are a million write-ups on how to do just about anything to ur e36 m3
here is a checklist that u should definitely check out. its e36 m3 specific:
http://www.e36-tech.com/forum/pre-purchase-checklist-for-e36-s-611.html
u can use the maintenance items not taken care of on the list as leverage for a lower price and just change things out urself.