Tip for anyone else planning on doing a conversion, milesvp mentioned in a thread a few months ago that other parts of the bike may likely need to be upgraded as well (brakes, suspension, ect):
https://news.ycombinator.com/item?id=28270121
I would suggest a steel frame with disc brakes. Suspension is a nice bonus. If it can carry a fat person it can carry a healthy person with battery and motor.