Are you going turbo or N/A? The 9A pistons will give you an 8.5-9:1 compression ratio, great for turbo, but you still won't gain much from keeping it N/A. If you aren't going for a charger then factor in some good cams, or at the very least a euro intake cam.
If you are OBD 1 you should be able to use the g-60 or auto 16v throttle body, otherwise you will need an adapter plate to fit your aba throttle to the manifold.
You left out a 16v fuel rail, bbm or ross machine racing have them. you will need digi injector cups to make your injectors fit.
If you are staying N/A then get the ABF pistons that I posted on the previous page. The distributor block off plate comes from a 16v engine, the one TT sells is to block off your breather, not your distributor.
Use you A3 accesories, but you will need 6mm shaved from the serpentine pulley. I have seen some people weld or machine the 16v intermediate timing gear to fit on the 8v intermediate shaft, saves having to change the oil pump and intermedait shaft, but leaves you with an extra distributor.
For N/A gains try and get a 50mm euro manifold or an ABF with fuel rail and throttle body. for turbo a short funner will be better.
You could do this swap with 400, but it wouldn't either A.) be turbo and B.) gain you very much if any real power.