anxilery wise its pretty simple:
If its from an audi you'll have to swap everything over, including the sump, brackets etc.
If its from a VW you can swap if you like, or you can keep whatever is on there and use the belts for the car it came from.
Extra bits to note:
For the 2e engine, if you have a digifant 8v already it will go straight on & plug in fine. If you have a k-jet you'll have to swap on your old k-jet dizzy with the 2e gear on the bottom and an adaptor from TSR to make it fit. It also has a breather on the front which you can either blank off with a plate form TSR, or fit & use the OE 2e breather system.
For the Audi 3a, I'm 99.9%sure its a k-jet lump, in which case if you have a k-jet it will go straight on, if you have a digifant you'll need a digifant dizzy. However in this case I'm pretty sure the standard GTI dizzy will drop straight in.