they never made a mk3 turbo.... well, except a diesel.
if your wanting a turbo, have a look at the recent PVW (april 2007), there is a yellow mk3 in there with a 2.0 8v turbo, i think abt did it, your easily looking at a few grand, (maybe as much as 4 or 5k) for the turbo conversion.
then there is the engine conversion route, you could get a full 1.8 20v turbo engine from the mk4, and drop that into your mk3... but then your looking at £1500 just for the labour, and the guys i was talking to about it are saying u need to spend a good 4-5k on the engine alone to make a truely (250bhp) rapid mk3 which actually goes round corners. you could do the conversion for cheaper, but u get wat you pay for!
and then u get onto supercharging, which is a totally different story