The turbo's are cheap enough on egay, but its the work involved fitting it that is the cost. I just had a quick scan, but cant get a price of a turbo conversion in the uk, but its $3k in the USA to give you an idea.
At the end of the day, like many people on here have said, what is the point in you making something which VW already make.
Check out Egay for a second hand 1.8t engine, but to get it fitted aswell, I think £800 is a bit ambitious.

Sorry mate, not trying to p@ss on your bonfire, just stating the fact.
Like I said, i would go for the wheels, exhaust, filter option and then at least it will lookand sound a hell of a lot nicer

Good luck