I would measure the inside frame make sure the motor will fit this is the thread for it: http://motorbicycling.com/f30/engine...cle-25074.html
I see there are a few threads on walmart bikes and a few bikes made with them. if you want it get it. just make sure on the measurements and i would see if anyone has put a motor on one and ask them about the mounting but it looks pretty standard. I am a novice on these myself and usually get a donated bike frame (someone is always upgrading their bikes and getting new) or find something for around 20.00 - 50.00. If you want something nicer you can always search out on CL for a decent GT, Trek, Specialized, or other known names. I know it is an exciting thing and I am always on a search for something. Its like being a kid on their birthday and getting exactly what you wanted.