Peter: Ever try to drive around a W140 in those streets? That is why they have standard electric fold outside mirrors. And even with that feature most of the times you are closer than you might want to.
Alon: the base mounts for each mirror are completely different, so replacing a square passenger's side mirror with a rectangular driver's side mirror seems like a complicated job. If you changed the whole passenger door with a R.H.D. driver's door it could done in a snap, but costwise is not very smart. Finally, you may convert your W124 driver's side manual mirror into electric very easy. As usual, Mercedes-Benz has an aftermarket retrofit kit that includes all parts (including a new electric mirror). Although, beware: it is not cheap!
Christopher: Right and left hand mirrors have to be different parts, regardless if they are the same or different shape.