Lack of use is a killer – never drive an M1 that’s sat in a collection for years without first replacing the £2000 dry sump oil tank.
Rust in the spaceframe structure around the bulkhead and sills can be a problem but is hard to see due to the full-width undertray.
Likewise, brake and coolant pipes can rot unseen inside their central tunnel. Again, an endoscope camera is the best check.
Check for cracks in the thin GRP bodywork, often found in sharp corners round the engine bay and headlamps.