hi abc,
before you go buying a bicycle make sure you do the correct measuring for compatibility with the gebe system. This is the most important part of the process as it is critical you have the space between the current spokes and the rear end frame.
http://www.bikeengines.com/bikerec.htm
this link tells you all you need to know about the right sort of bike to choose. Once you understand the basics of what the bike needs, go shopping, as far as bikes go the Sky's the limit these kits can be mounted or various styles so long as it is compatible. You should be able to find a bike under $200 no problem.
Wow your gunning for it straight out of the blocks with the 40cc i chose the 33cc after reading many reviews and builds here, so you are in the right place to find out about builds, just do lots of research and your build will go smoothly. If you check out the very first thread in rackmounts (golden eagle support services) thats augidog (a once member) who now works with the company and will help you if you run into major issues that none of the fellow members can help you out with.
bamabikeguy posts have a wealth of info that will help you, and once you start your build, remember to document it and if you run into problems i can almost guarantee you will have a reply within the hour. Trust me i have been down this route and the fellow members on here are great.