Yea, I'd lean towards Friction Drive(FD) too. When you're not using it, just lift it off the tire for no drag what-so-ever. With an in-frame kit, chain drag when just wanting to pedal is a nuisance.(or when she inevitably breaks down)
FD engine options are plentiful, just remember, the more hp ~ the more gas she drinks...
has a nice FD kit that alot of engines will bolt to. and you can get different sized rollers for either top speed, or climbing powa.