16V conversion is apparently easy but not a major power gain
As you've mentioned, easiest and cheapest conversion if you like to scream the nuts off an engine to get the best out of it.
20V Turbo engine is very popular and delivers good power?
Best all round choice out of all the engine conversions IMO. Light, reliable, easily tuneable and readily available.
VR6 is very good power gain but is very heavy and a difficult job?
Great engine with a lovely sound and plenty of grunt, but too heavy and totally ruins the handling on a MK1 - It sure ruined it on both the MK2's I converted.
G60 Supercharged again seems popular?
Again, much like the 16V conversion, the G60 is a relatively easy one and light enough not to spoil the MK1 in the corners department. But the G-Ladder chargers can be 'fussy' at any sort of decent boost if not maintained and built properly, and specialists for them are thin on the ground.
Here's a what a 20V turbo looks like in an MK1 that Awesome GTI did.
http://www.awesome-gti.co.uk/images/mk1tt3.jpg