1. 1.8 driver lump will drop straight in, the 1.6 & 1.8 lumps are physically identical, VW just dropped a different crank & pistons into a 1.8 lump reduce the CC.
2. Issues with GTI head are as above, injectors and coolant channels, as said leave the injectors in or remove the inserts and drop coins down the holes and thread em back in, I've heard 1ps fit, or was it 2ps?
3. Yep the 2.0 will bolt straight in, just swap the engine mount brackets to the 2.0 lump. Issues with this are the carb fuel pump, I don't think it will bolt to the 2.0 block, and the dizzy mounting hole is too small so you need an adaptor off TSR. You also need to swap the 2.0 dizzy gear onto your carby dizzy.
I'd also fit a 1.8 K-Jet GTI dizzy rather than the carb one, the advance curve will suit the GTI head/cam better.
4. Auto gearbox will bolt straight up, just buy the right clutch kit and bolt on your old flywheel. Just make sure you mark it with a 6degree BTDC mark, the carby 17degree mark will be too retarded to make the most of the GTI head/cam.
Haynes has the procedure, basically you measure a set distance from the TDC punch mark and put your BTDC mark on
