At the minute, its possible to get hold of a tidy TT for £5k; as mentioned, you would probably be better off buyting a TT.
As well as the cost implications of converting an N/A to TT:
You'd have to consider the labour costs (if you're paying someone to do it for you), the quality of the work carried out, the administration work - ie informing the DVLA, your insurance company etc.
Then there the quality of the transplant engine; not knowing how many miles it has covered, or whether it runs as it should until it has been fitted.
If you're going to be keeping the car as a manual, you will need a decent clutch to tranfer the power to the N/A 5-speed gearbox - but staying with the 5-speed will limit you to what performance modifications you can do as it is not as robust as the 6-speed or TT auto. If you did want to upgrade the gearbox, you'll have to budget for either a quite expensive (about £2.5k) 6-speed manual, or the cheaper auto box.
You'd be better off finacially if you bought a TT, plus you wouldnt have any of the hassle that goes with the conversion