with it being early the fusebox is different to mk3, you could fit a complete 90spec mk2 car loom or just splice pre-90 plugs onto the abf loom. its not particularly difficult but i can do conversion looms if you arnt keen./ you dont HAVE to swap the beam but fitting one with a roll bar in from a gti is a nice upgrade. you could just bolt on an aftermarket arb though which would be just as good. no rear discs but you dont really need em drums work just fine. up front need to change pretty much everything bar subframe and wishbones, even brake servo needs swapping for a 9" one. nothing thats particularly difficult, and if you get a donor car as above makes things easier. complete 16v would be a nice place to start would give you everything you need including fuel setup, though would need to go 5 stud if you use its complete running gear. depends if you're bothered about that or not!
abf -> mk2 is a bolt in thing using all off the shelf vw parts if you buy the right bits, only thing that isnt direct swap is the wiring loom