Re: Engine kits
I'll repeat the advice above; read, read and then read some more. Just click on any thread headline that looks interesting. Then read til you've finished, or lost interest, and then click on another one. In no time you'll have a notion of what sort of engine/drivetrain you want.
You're bound to have fun. There's sure to be some frustration, too. But that's okay; once you've gotten to know your bike you'll become more and more confident and you'll start clocking more miles between getting out your tool kit.
As far as speed, I'd recommend between 10 and 20 mph. These bikes can go faster. Plus there are folks here who seem to be pretty knowledgeable and seem to get away with faster speeds. But I just don't trust bicycle wheels, bearings, frames, etc. at such speeds. Even higher quality stuff.
My own feeling is that a motorized bicycle should not be viewed as a mini-motorcycle or even as a scooter equivalent. They should be viewed as a highly competent bicycle.