[ecomodded]

I would test drive it, listen for obvious faults, it should not be making any other noise then the constant smooth sound of the motor running. Anything else means something needs to be fixed. Wheel bearing are a common part to need replacing on the VW's, check for them by turning the steering wheel from side to side abruptly to put them under load. Any repairs you can find now will help you negotiate a lower sale price. Cars need maintenance, some people just drive the crap out their cars and neglect the maintenance of it. Check for a clean air cleaner topped up fluids and general tidiness of the car.
A common problem that the tdi's have is that their intakes get clogged up by the EGR valve, restricting the inlet pipe size to the size of a dime, in some cases. Symptoms of a clogged intake pipe is barely any power past 3,000 rpm.
I recommend taking it to a diesel shop to have inspected.
Buying a used car can be a gamble, more so if you do not know mechanics and don't take it to a shop for inspection.
I would not take it to any shop, a diesel shop will be more knowledgeable as to the motors condition.
Also remember that the standard transmission gives the fantastic mpg. I would pass if it was a auto..