Check the service history for water pumps, make sure it has had at least 5 or 6 installed over a 10 year period (if it's a 95 reg)
Stay on top of them, if they blow whilst driving, they piss water everywhere very quickly and that aint good if you're on the freeway or somethin.
Also, biggest issue with E36 M3's is the vanus system. When you drive the car, make sure it doesn't "chug" or struggle to accelerate, as if something is slipping.
Test this by holding at about 3000 rpm in second, and slowly accelerate. You'll feel it chug. If this happens, get it checked over by a 3rd party mechanic before buying it. It could be the clutch or as little as the fuel pump fuse, but if it's the vanus, DONT BUY IT
Other things such as rust hotspots, above the windscreen and rear wheel arches tend to bubble a bit, This needs attention, but no more than $2-300 to spray over and treat.
Other than that they're very resilient cars. Remember if it's old you may not be THAT impressed with the performance. They are a sports car after all and probably been thrashed about.