If you're buying a used P-Car (or any exotic I guess), I recommend you either get it from an operation with a strong reputation to lose (e.g. OPC - but they are v expensive - or a well known independent), and/or find a used P-Car expert who can crawl all over the one you're actually buying to check it out.
Getting as much info on the model of the car you're buying as you can might be a good idea, too - this board's a pretty good place to start, I guess. The Porsche GB Owners' Club, which is quite cheap to join, also has a wealth of info available and a huge network of local P-Car owners who I'm sure could provide loads of support and advice.
Although I hope the misunderstanding over the 3.6L vs the 3.8L engine spec by your dealer was unintentional, this is an extreme version of what could potentially go wrong.
Hope this somewhat granny/eggs post is of some use; good luck with your purchase - you'll love the car however you decide to get one.