I am a very happy owner of a 500 C/M. I used it for quite a few years before I realised that it needed a service. It's a 1977 model, and it is quite possible that its service a few years ago was the first. Had I known what a Hasselblad usually sounds like, I would have known on purchase that it needed some work. Anyway, after I got it serviced a few years ago (by an awesome Hasselblad service man in Adelaide), I realised what Hasselblads usually sound like and feel like.
As others have said, do the usual checks on lenses etc, to see if there are any scratches. Also, be sure that any inserts match the backs (same numbers).
Other than that, enjoy. It is an awesome camera.