From how I read it he is buying a car that is already turbo'd.
Really turbo systems aren't that complicated, just more moving parts. I would say make sure that you take off the air intake pipe and check for shaft play in the turbo. There might be a little wiggle in it but you don't want loosey goose ya know? Perhaps google it if you want to see examples of shaft play.
Also look at the fins of the turbo while you have the air intake pipe off, see if they look nice and straight. If they are chipped or bent, that's bad! Make sure there is a proper vent and/or catch-can setup. Other than that it's the basic stuff of checking for oil leaks and exhaust leaks.
Many turbo cars have an oil pressure gauge, check that while you are driving and getting into boost a bit to make sure it's fairly steady. Also watch the boost gauge and make sure it doesn't get too crazy during normal accel.
