Right as nobody as helped you out on this i will!
The turbo on a s3 (apy or apx lump) is a KO4, you will need oil and water lines, new exhaust manifold and downpipe as well as the injectors, then you'l need a remap straight away. You can get away with using a standard mk4 clutch depending on what power your going to be running, I have heard of alot of people using clutched from a vr6 on 1.8t's with ko4's.
If upgrading from a KO3 to a KO4 I wouldnt get a flash map done (generic map), I would get some dosh together, take your car to somewhere with a rolling road and get a live custom map done, its the only way to really get the most out of your car safely, rather then just having a presaved map which isnt designed specifically for your set up.
hope that helps mate!
Ash