I can see that back button focus isn't going to work like the original EOS-M, which I had and liked but traded the system for a backup 7D body.
I use back button focus on my M. Why do you think the M3 won't have that capability?
$600 for the camera plus $150 for the EVF plus $50 to $100 for the adapter? It's getting expensive even at $600.
Yes, you could set it up but the button is not in a good position, for me anyway.