I presume you already have an 8v gti then? If so the usual swap is to use the AGG/2E bottom end, refurb your current cylinder head and fit it to the 2litre block using the AGG/2E headgasket. Then use all the existing ancillaries off your current engine and drop it in - straight swap

Do a quick search on your favourite search engine and there's loads of info on the distributor mod you have to do, but very straightforward.
If you don't currently have an 8v gti then you will need the whole lot from the mk3, wiring loom included. I presume your car is CE2 (is the hazard switch on the column?) so the wiring will pretty much plug straight in. Again, easy swap, but if you're doing a full engine conversion then you might as well go for something with a bit more poke, like an ABF. If you don't like 16vs then stick a G60 in it
Edit; If you currently have a carb engine, either 1.6 or 1.8 then you can grab yourself a webber carb and block the injection holes in the head, think a 1p coin is a good fit. Not going to be as fuel efficient or powerful as a full injection swap, but should give a bit more poke for little effort.