21K for a 96 M3 from a dealer sounds OK as long as they offer you some reasonable warranty. You could probably find something under 20K (95-96 M3) but expect higher mileage. My brother has a 95 M3 with UUC roll bars, exhaust, H&R springs etc and it is simply amazing to drive. One day I will do that to my 97 328i. Also it depends on what you're looking for. If you want a pocket rocket at a low price, look for a E30 M3. On the other hand, if you want a moderate mileage 95-97 M3 with the lux package, expect to pay in the mid 20s. Can't go wrong either way.
Have fun
Paul
97 328i
87 Carrera