When I went to the recent NO RICE meet there was a bloke there who had a Golf convertible with a 20V audi S3 lump in it, he said he had spent about ?6000 on it. He had it fitted by volks-speed.
Getting the engine into the car is the easy bit as the mounts line up ok, the wiring and setting up is going to be the hard part, A lot of them have the anti theft units fitted so you will need to by pass it with a dedicated ECU.
try the Volks-speed site as they seem to be the best around at the moment in making these conversions easy.
www.volks-speed.co.uk