you can get a VR6 engine reasonably cheap. I priced up a 1.8T converson for my old mk2 valver, then sold it for a 93 WRX for less than the conversion cost. The VW turbos are good for big power (there's plenty of 500bhp+ ones about), but not a cheap option for a track car.

Loved my old Golfs, but I have to say the Scooby's is a different league to drive, and much easier to keep in a straight line