The E30 M3 community has come up with a "$15,000 rule" when it comes to their cars. Basically, you will end up spending about $15k to get a car in good shape. Most of the higher-dollar cars are already in good shape due to diligent maintenance and important repairs, and the cheaper ones will require a little TLC to get them back in good order.
It seems to work out. The more expensive M3s out there have lower miles and have had a lot of the significant work done to them.
Conforti chip, Evo airbox/K&N filter, UUC short shift kit, oil pan baffle, Hella H4 lights, OEM glass sunroof
sold: '99 M3 Coupe
Airbox baffle removed, ZKW ellipsoid headlights, UUC short shift kit, BMW x-brace, debadged
'96 Jeep Grand Cherokee Laredo V8