Re: how much is enough?
A stock China girl motor should be able to pull you at 20 mph. You will have to pedal a bit from stops to reach cruising speed. If you live in a hilly area, a jackshaft/shift kit will help more than a larger motor.
Some of the rear friction motors like the Subaru are powerful and reliable but may not be suited to a damp climate.
“You're only young once, but you can be immature as long as you like" -Unknown