After 25+ years of shooting 35mm, I got a M645 last month, so take some of my experiences with that in mind. But I did do some of the research you've done recently.
Originally Posted by slnce-z-gsi
I can only answer your last two:
You can't use AF lenses on MF bodies (only the other way around)
The M645, 1000s, Super, Pro, and Pro TL have MLU. The J and, IIRC, the e don't.
The M645, 1000s, J, and I think the e don't have interchangeable backs, only inserts.
I don't know too much about the e - they have the same features as the classic ones, at a higher price - and they are missing some of the features of the pro and pro tl.