I have just bought one about a month ago a 07 3.4s
Things to look out for as mentioned by others
Service history ,goes with out saying on any sports car
Rear oil seals haven't gone , telling sign oil deposits inside the exhaust pipes or burning a lot of oil ,lumpy idle.
Bore scoring , endoscope to check , a good independent can do this as part of a pre inspection .
Condensers( there 2 of them ) about £60 each and not to difficult to change , bumper off .
Trye wear , there are very sensitive to geo set up , make sure tyres are good and of a Porsche spec .
Spec ,try and get the best spec possible , I had a wish list of about 6 items I wanted
Sport seats , multi function steering wheel , sat nav, climate ,short shift and 19†alloys .manage to get 5 out of the six didn't get the sport seats but I have the comfy seats with the bonus of Porsche embossed which was a extra.
Also wasn't my first choice colour but third behind grey and red .
Other than that there are bloody good cars .
Any more advice just give me a pm .
Also if he gets one first port of call is to get a Remus cat back exhaust ,than it sounds like a gt3!