Surprises me people are suggesting trying to get a refund
As I posted after 2,000 miles bottom end + turbo went on mine which cost me £600 with all the bits and mates labour to fit/fix with the belts etc...
Anyway I never even considered taking it back when the smoke came out to be honest if you buy private that's it really sold as seen. If it was a dealer I would suggest otherwise but once you have driven away the seller doesn't have to do anything.
Hope you get it all sorted though!

Must say your total cost seems high for just a turbo I got a bottom end, belts, turbo, headgasket, waterpump all oil filters and coolent for about £500 looking around then paid a mate £100 to give a hand fitting and donated my ko3s in need of a rebuilt and its still running sweet 8k miles later