I almost bought one of these in 1985!It is called an M635csi(dont think BMW ever labeled it as an M6?)

The 286 BHP engine is very robust.Its an evolution of the M1 engine(277bhp)and the latest evolution engine can be found today in the 360 Bhp M3 CSL, so you shouldnt have a lot problems with this.Just check for leaks around the head gasket.Depending on the mileage,check also if the Timing Chain has been replaced(100,000miles ++).
Never heard of any bodywork defect issues.Generally these are garage kept cars.