Hi Nick, just a thought...as I too did a bit of research when I was looking around at 968s. The engine (as you are aware) is pretty much the same as the 944 S2, but Porsche added the Variocam device in place of the cam chain tensioner. This pushes the cam chain by hydraulic pressure and varies the cam angles when at certain revs. However, this action does tend to induce more wear on the cam sprockets and 968s will need to have the cams replaced once the teeth have worn badly - this is not cheap!
So, best to look for a receipt that says the cams have been replaced in the service history.
I also understood that although the 968 doesn't have the same sill design as the 944, those plastic trims can hide all sorts of rust problems and repairing a 968 can be a little more difficult that a standard 944 due to the body design.
I am sure that other 968 owners will chip in with their comments too, but good luck with your search.
PS - Didn't get one in the end...but I really hope too some time in the future!