Failed turbo bearings (resulting in needing a new turbo) are extremely common, rev the engine, let the revs settle and if you hear any howling noises (some say it sounds like an owl or a police siren type noise) then walk away.
http://www.mk5golfgti.co.uk/forum/index.php?PHPSESSID=fm4hovg134985l6userik79770&topic=28879.0Here is a link to my GT tdi in my opinion it is one of the nicest examples (silky smooth ) out there