Having been 'at the sharp end' for the better part of 5 years, I have some advice for you.
3 words, actually.
Shimano, Bosch, Yamaha.
If you insist on a conversion the only one I would consider would be Bafang - mid-drive only. If you're at all sensitive to q-factor then that's out, too.
I have come to the conclusion that wheel driven motor conversions are this:
UNIVERSALLY UTTER SHITE.
Seriously, don't bother. I've stopped selling, fitting & servicing them.