Engine, gearbox, most likely rear diff, wiring, ECU, fuel pump and exhaust.
You should use an ECU that matches the engine you use.

+1 for a half cut...

in my efforts yesterday tracking down a black box ( Cheers BEN ) there was multiple TT half cuts fairly cheap all around the country , ranging from 1100 - 3500 includuing most things u need ,  if i was you id sell the rx

if your rx is not too modified go for the 4SALE sticker mate and buy a B4, that way your already getting a car with bilsteins and semi desent brakes and leather and a sweet sound system , try and buy one without a MRT tune onboard and get someone to flash her up , then gradually add your mods and retune to suit ,

either that of buy a RS

Cheers guys, some great advice.