The rack / carrier mount isn't for everyone but some people do like and prefer it. Personally, I don't...I prefer to have the engine sitting low in the bike, it's easier for getting on and off, and provides better handling, or so I find, with having the weight of the engine sitting low.
The in-frame design that I sell, commonly referred to as a "happy-time" engine can not be easily (or safely) installed into a woman's style of bike frame. If you are thinking of going with this style of engine, I would recommend going with a men's frame. The smallest frame size that you can "easily" install this into would be a 19.5 inch frame. You will want front suspension on her bike (not needed but is worth it), you will want the bike to have what is referred to as a hard-tail suspension...basically, no rear suspension.
When you go bike shopping take a tape measure. Hold the end of the tape on top of the crankset...extend the measuring tape 16 inches or so up the seat tube and sweep it forward to the front downtube...if you have that much room available without hitting anything you should be okay for fitting the engine in there.
To get a good idea of how one of these will fit into a bike take a look at some of the bikes in the photo gallery on my web page. Here is a link to get you there http://www.chrishill.myknet.org/photo.htm