It would cost £k's to do the transplant. If you added that money to the sale of the GTI you could get a S3....
What about running an R32 block in FWD? Might be less work to configure that as you wouldn't have to mess about with cutting the floorpan and fitting diff, driveshaft and haldex controller
4WD over-rated in my opinion. better to save up for a golf r or buy an s3 methinks.