Don't know a great deal about the diesels, but that looks like a nice example, fairly priced too.
Apart from the usual things you check when buying a new car, check in the history to see if its had the window regulators replaced. The nasty little plastic clips break on those and your window disappears into the door (just ask kniterider). VW will give you a 50% discount on the labour to repair these as it's a VERY common fault. Not common enough for them to do a recall though apparently...
MAF sensor failure seems to be an increasingly common fault too, but thats not a huge problem.
Finally, if he still has it, buy ant-gti's tuning box to stick on the car when you're driving it, that'll put a smile on ya face.
Chris.