You need to make sure the engine is in great working order, the m/engines require more maintenance, and if you need repairs expected it to be very expensive. Make sure service records are up to date, also need to test drive at 50-55mph to make sure the front suspension is ok. Look for a shake in the steering at those speeds. It's usually a part of the car that needs attention. Really need more info to give a price estimate. M5s in great shape usually fetch 13-15 grand, above average cars are around 8-10 grand, average cars around 5-7 but they may need more work.
Sent from my SPH-M910