Old rule of thumb:
Will the motor fit in the nose?
Will the plane balance with the motor in the nose?
How much battery can I shoe-horn in the available volume?
How long do I want to stay up, at full power?
How much Magic Smoke can come out, before the plane doesn't fly any more?
How soon, before the wings tap tips?
(I'm a NAUGHTY boy!
On a serious note, everything you state there, is a good point.
When I ventured from gas to electric, I had to do a LOT of question-asking, and cut & try experimenting.