How much money you got?
ultimately thats what it comes down to.
a pinto will be your cheapest. cheap parts, heaps of info about mods, not much cutting up.
4age, find rwd sump etc, you going to use a t50 box, good luck finding one. plus all the wiring, ecu for injection and stuff. unless you carb it?
12a, prices of rotarys and going sky high atm. ok power standard, great when ported but fuel could be an issue. gonna have to mount it on an angle to clear the steering rack. some cutting up for the box i think.
sr/fj, ecu etc. wouldnt be a bad conversion tho.
you just gotta weigh up the pros and cons of each conversion, will it be a daily, also dont forget upgraded brakes, suspension, feul system etc.
good luck anyway