I think the rationalization for the appearance thing is that they didn't want the robot version to know it was a robot until they revealed that fact to him (though the TV reason being that they didn't want the audience to know at first either). William mentioned that there were issues with the mind or body rejecting itself (they weren't sure exactly why), and maybe being in a body exactly how you remembered your body being was supposed to help that. And once they mastered that sort of consciousness-transfer, I assume they could create other, younger or entirely different bodies ad infinitum.

As far as William's motivation, it was basically the old man's reason to leave the company to him. And even beyond that, I'm sure William considered it an interesting experiment with far-reaching consequences if they succeeded.