Got to be 16V, even if just for the integrale heritage. The lack of horses isn't going to make much difference in the long run as I can't imagine someone who goes to the trouble of a 4x4 conversion is going to keep the car standard under the bonnet
I've never bothered even chipping my 16VT because I reckon its a total waste of time on a FWD car, if it was RWD or 4WD I'd change my mind.
Make the conversion as cheap and simple as possible and save as many coupes as you can so you can get a bigger market and prove the concept. All you 20V owners should keep your nice 20v examples mint and buy an old 16V for the project, they're not exactly expensive!
Then when we're all happy with the idea of the 4WD coupe and there's some experience of the issues involved - do a kit for the 20V afterwards!