Welcome to the forum, glad you joined us
I've never seen a woman's frame bike with a center mount motor, not saying it can't be done. Most who use a womans frame use a rack mount motor.
What kind of bike is suitable bike to modify? Any bike that will fit a motor, center mount need's the room to clear the motor, rack mount will work on almost any bike. I would ask yourself what bike do I like. Here is a link for more info
Street legal? I'm not from Illinois so I really dont know
Illinois Rules of the Road
(scroll down to moped's
Where to get a kit: I've gottne seven from KINGS MOTOR BIKES,MOTORIZED BICYCLE,MOTORIZED BIKES
and have had no problems.
I've also had a few from Spooky Tooth Cycles - Motorized Bicycles: Gas Powered and Electric Bikes - Home
and as far as I know they are still running great.
ThatsDax is a sponser of the site, I have not used him but I hear the service is awesome! bicycle engine kit, bike engine, bicycle engine, bicycle motor
Gas or electric
Electric cost more, they are havier, limited range, Don't know if it will reach the speed's you are looking for. Don't know to much about them
Gas bike's look cool, they are cheap, as long as you got fuel you can run as long as you like. Alot of maintanece.
I hope I was some help, other member's should give thier two cent's