Audi S3, Golf R32 (mk5) of R20 (mk6) and Leon Cupra R front brakes all use the same caliper (in different colours) and use a 345 x 30mm vented front disc. They bolt straight onto the standard GTI hub and if you're running standard alloys, you'll have no issues with wheels.
You can normally pick up a whole front end swap for £300-500 depending on the condition. Best advice is to buy calipers, shields and lines used but new discs and pads (unless very low milage).