The 2005 through about 2010 Phaeton had problems with roof rails, wet bay, and the front DS slide floor problem (water). Check to see if these have been addressed.
There is nothing wrong with the Cat C-7, keep the oil changed max 5000mi and change the fuel filters ever other change because of the Heui pump (High-Pressure Oil Injection Systems)
How old are the tires, any fogged windows, is the radiator clean because of the slobber tube. Does it come with records, spend the bucks and have it checked out may save you in the long run.
2 stroker