Re: good(cheap) bike to start with? wally-world?..
I usually find my bikes used on craigslist. Like it was mentioned above you can find some really expensive bikes with all the good components for really cheap prices. I don't ever spend over $50 and have purchesed some 3 to 4 hundred dollar bikes. I also watch for the Schwinn OCC bikes, but they are a little more of a challenge to build.
For your first build you may want to find an older style bike with the small frame tubing. Much easier to build for your first one. Motor will bolt right on without making motor mount changes.
Radius on motor is designed to bolt to 1 to 1 1/8" frame tube.