Hi mate, I done this conversion to my 8v corrado as I had a golf mk3 16v which rotted from under me and the 8v was dying in itself.
It is a straight forward swap and in all honesty the worst part is fitting the engine loom to the cab. Also I had to fabricate an airbox as the mk3 airbox doesn't fit at all. Otherwise get a donor and grab the engine loom, transponder ring, and gearbox although the ratios are the same as what you have now, can't remember if the clutch is bigger on the 16v though. Grab the downpipe and exhaust to the cat too. The cat on the 8v is the same as 16v but lambda plug is different. May as well grab the bigger brakes 5 stud etc too and abs system as its all much better and easier to fit whilst the engines out.
Minimum is the engine, rear engine mount, engine loom, downpipe, Ecu De immobilized or with transponder ring and loom if not and the key obviously. Otherwise 8v stuff bolts up the same.
In all honestly the 8v had a much nicer pull to the car through normal driving but as you said gutless after 4k The 16v is more powerful granted but has a much poorer low down torque for everyday driving in my opinion but over 3k comes to life. Mpg is better on the 16v for me too which was a bonus.
Any specific questions let me know and ill try and help. There's a good guide on club gti about installing an abf with digi 3.2.
Craig