I don't know if there are any 'COMMON' failure points with these cars. I would say first, check over everything that is supposed to work on the 2001 C4 Cab and how it works as I should imagine there are more things than on the boxster. More things can go wrong with a more complexed car. Read up 1000 things you didn't know about your 996 on the rennlist discussion forum at the top of the first page.
http://forums.rennlist.com/rennforums/996-forum/530215-the-new-official-things-you-didn't-know-about-your-996-thread.html Lots of little things that could be overlooked on a PPI. ie:- inside lights, alarm, petrol door, parking lights, 7 micro switches in each door controlling differant things. Little things can be missed and could add up to a huge amount to fix unless you are a handy DIY man. Forget the obvious wearable things like brakes and clutch and tires. They speak up for themselves. Use your eyes and ears but first know what you are looking and listening for. Take the car for a good 30 miles run before pulling the trigger. Does it have the hard top with it? Does the top open/close via the fob and/or dash button and the door lock? This is one case where you defo look and listen before you leap. Good luck with your purchase.