Tips on buying a '95 M3:
-Get a compression check done. Over-revs were common on the early cars because ham-fisted drivers would pick the wrong gear when the soft tranny mounts flexed enough to move the gates. Uneven compression will show any valve damage due to this. It will also give you a general idea what shape the engine is in.
-Have a good BMW shop spend a lot of time looking at the suspension. Especially the rear. Early M3s are notorious for going through rear shock mounts and toe bushings. Shock mounts aren't bad to replace, but if a worn bushing allows the rear control arms to rub against the metal carrier, this means body damage.
-Make other sure common wear items like the water pump and radiator have already been replaced. Fuel pumps are also problematic on early '95s.
-If the car looks too good to be true, then it probably is. Check for over-spray, bondo and mismatching panels. Each BMW body panel has a VIN number stamped on it. Replacement panels will show a "-R" at the end of the VIN. I once looked into a '95 that had an unbelieveably nice Mugello Red paint job. Upon closer inspection, I discovered it had been wrecked. Twice.
-Follow the same tips you may get for buying any other used performance car. Make sure you get records and books if you can. Do a title search. Have a good mechanic go over it with a fine tooth comb. A GOOD BMW specialist should be able to look at the list I just made and immediately recognize all the troublespots.
-Be weary of "M3 conversions" from 325s or 318s. Done right, they are good cars. Done poorly, they are worthless.
(There's probably a few other little things I forgot.. it's been a while since I made a list like this... I'll add them if I remember any more)
There are lots of good ones out there. A well maintained M3 is one of life's great pleasures. Good luck on your search.
M