[DaveH]

1) you need the downpipe, since there isn't a stock one you can use.
2) staying around 300hp you might as well go with the TT sidemount. It works well and is cheap.
3) get a steel headgasket, toyomot, powerhouse or Fmax sell them
4) egt gauge tell you roughly how your fuel mixture is. too hot means too lean, too cole means too rich.
5) the AFC is a excellent fuel controller, I have one and like it a lot
6) the turbo timer just keeps the car running for a set period of time after you remove the key. It lets the turbo cool down. You don't need one, you just need to sit there for a minute or two before shutting off the car.

 

[DaveH]

You can have a cat put on the downpipe and it can be emissions legal. The stock tt DP won't work either, as it is setup for the stock twin turbos.

I would definitely look for a mechanic who has installed a turbo kit before, or at least a mechanic who has done custom work on imports before.

If you live close to me drive it over and i'll help