First off 20's DO fit an e30. It has been done. You can't really do any suspension stuff however with a rim that big. I think he's doing the right thing staying with 16's as a larger rim really does slow you down. (wanna talk physics?
I'm curious as to what type of suspension mods your gonna do? Thats alot of weight upfront. Also are you going with a different brake setup as well?
Off hand anyone know exactly what engine that is so I don't have to go look it up?