I don't have a passenger mirror, the drivers mirror isn't exactly effective, not compared to modern cars, hence why i've never considered fitting one on the passenger side
If you do fit one, best position it to suit your driving position, ignore any dimensions you may have read, every driver probably needs it in a slightly different position, and you'll need the strengthening plate on the door shell, so not a quick job
You've considered a reversing camera, these can be wired or wireless, or you could think about a dash cam that can also be used as a reversing aid? Best of both worlds then, dash cam in case of an accident plus help when reversing
Lots on the market