mileage, condition, maintenance history, engine condition.
The rule of thumb is that an E30 M3 will cost you at least $15K. If the car costs less, you will make up the difference in getting things right.
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)