It's always down to the buyer to carry out those checks, unless you can get them inserted into the deal i.e. you pay and if they check out, the seller knocks the cost off the car. You should get a respected company to do the PPI (pre purchase inspection), I'd personally use Peter Morgan or Mike at Sports & Classic on the mainland. Don't go on if they've been done previously unless very recent, like last week. These things can be expensive if they go wrong, so make sure you're happy first and foremost. You should be looking at around £200-£300 depending on who and where.
Look at it this way: You could buy the 911, run it for a year to say you've done it and if you don't like it, flog it with minimal depreciation and then buy the Cayman.