1) Which donor car is best to pick up? MKIII GTI or one of the Seats with an ABF?
Cordoba as it has the right length shafts on the 'box and is cable clutch.
Toledo as the loom is most like a Golf2 and it has the full cable clutch set up.
Golf3 because it has a VW badge on the inlet manifold (and I know you're a tart).
I'd keep all the wiring and everything sitting on the subframe, pretty much. As well as that I'd hold on to the exhaust off whatever it is you get.
2) From said donor car does anyone have a list of what I would need to keep and what I can junk/ebay? (Not going wide track but want to run Digi III management to start off with)
The main thing is to get a whole car and use everything from that car, to avoid any PITA year-to-year, model-to-model differences.