I can't answer your question about the wheels, but i am in the middle of that dilemna right now with fitting 4 piston R32 calipers on my U12.
A 12.5mm spacer leaves approx. 2mm clearance between my wheel and the caliper.
I think that basically means you need a wheel with an offset of around +27.5 (possibly +30), my current wheels are +40 offset. With the spacer on, the tyre is only JUST under the guard.
These aren't free floating spacers either. They bolt onto the hub using my old wheel nuts, and the wheel bolts onto the spacer, as the spacer has it's own OEM Nissan front wheel studs. One prototype has been done, the design modified and i'm about to send the revised design off again to be machined up; hopefully they will be perfect.
So far in parts this brake conversion owes me just on $800.
EDIT: A pic.
