As said get someone with you who knows about these cars, even if you have to bung em a few quid.
Bodywork can cost big time, as can engine problems, rear arches can easily cost £500 to put right, sills rot, remember how old these are, why is it black, as said rare colour, is it using a respray to cover a shed of a shell ?
Don't even think about any offer until you hear the engine running, even one with shagged rings will sound great and drive great, as oil rings generally go first, so drive well but drink oil nearly as fast as petrol.
Maybe say to him you will give him what you think it is worth, plus the MOT fee, if he puts 12 months mot on it, he knows the FULL story, if it were just the lack of MOT he would MOT it and sell it without even really needing to advertise it.
Does it actually run ?
tabetha