Things I would ask depending on if your buying from a private seller or a dealer.
Mileage...
mostly highway or a mix of both?
Was the oil changed at the correct intervals
Check to see what brand of filters were used (Mahle/Mann, Bosch, Fram, etc..) OEM shows the owner really like to stick with "orginal" parts and paid the bit more for the better filtre (Mahle/Mann, Bosch) compared to an Autozone Fram, Purolator, etc.
What kind of mileage /tank has the car been getting? Has the timing belt been changed at the proper interval? Where was the majority of service on the car done, etc.
Alot of common sense things you'd normally ask about any other car. Big thing on TDI's believe though as with any diesel VW. is the timing belt change intervals and what kind of mileage the car has currently and what kind of mileage (meaning highway vs. city) was the car typically driven on. Maintenence is most important too... most I've seen have kept up their service records pretty good either through the dealer and the book that comes with the car for a record...or on a scratch pad noting dates for oil changes, servicing, timing belt, etc...
Joe