If I had the $$$ to throw around, I would own one in a heartbeat, regarded by many to be the purest and best handling m3 of them all.
The engine was only used in the m3, I don't remember the code for it, but parts are going to be hard to come by and expensive (even for a BMW). You have to keep in mind that this car was producing close to 100hp/l back in the late 80's. IIRC, the heads it used were similar to the design on the M1, being pretty advanced and powerfull, but once again, rare and expensive.
Any non-m3 specific parts should be fairly reliable, once again keep in mind that european car parts are generally not cheap, and especially for a car about 15 years old, can be hard to find. Once again, if I had the money I would gladly buy one and turn in into a terror for road course's. Unless you have deep pockets or are a BMW mechanic with good connections I wouldn't reccomend one for anything other than some kind of side project.
HTH,
Pete
