Like Brian said, it sounds like a good trade based on money, as in the M3 + 50 should be worth more than $900. Assuming of course neither of them needs to go in for repairs.

I say this as an RF shooter though: Not sure you will gain a lot in terms of image quality -- it's probably a step down going from 645 to 35mm. Though there is more to photography than sheer image quality, which is why I shoot 35mm