turbo

sounds tricky. You are going to have to study the manuals of both to seek out where differences between them are going to be the differnce between not catching on fire or putting that insulative shim in there (if one exists) ore bursting into flames. Sell yours, buy another with one would be my recommendation.

After-market turbo kits for the ABA 2.0 are expensive. I'm a firm believer in using the engineering expertise of VW to enhance my VWs. Black Forest Industries is doing a project you might want to look at: http://blackforestindustries.com/blog/2013/01/21/mk3-1-8t-drop-in-conversion/ the prospect of a reliable, VW designed solution for 200+ hp in a MKIII is pretty attractive...just sayin'.

for the ABA engine you can buy a header for turbo, buy a cheap turbo from ebay t03 would be fine and use a kit for the intercooler if you put no more than 8 to 10 psi you don't need to open up the engine. get some bigger injectors play with the maf etc. voila you have a boosted mk3. cheap

You can a wrecked MKIV 1.8t for $1000...just look for one hit in the rear...