Re: Brand new, don't know a thing, hope to change that!
The three speed will require a lot of grinding of the sprocket to get it to go over the hub.
That being a girls bike will make for an easy (but high) mounting of the engine, you'll have to figure something out for the gas tank mount.
A bike like that is best suited for a rear engine rack mount.
If you do go with the 'in frame' type of engine (the kind you were looking into) buy from a reputible company with good prices. Look to the sides of this page for suggestions.
I wouldn't shell out a bunch of money for an expensive version though, since you are mostly paying extra for hype and that's all.