Which E320i? The E30? I would also have gone down the restomod route but being a bit more biased to keeping the car as original as possible. Will the cars be stored in a garage with space that you won't miss over the next 2-3 years?
The E21, based on the same engine as my cars.
Everything changed will be reversible.
The drivetrain (box, shaft, and rear axle) is interchangable. No need to worry, the car can be turned into original in about an afternoon.
I have space for the two of them over an indefinite time in a fully equipped garage. All I really need to do is get this: