I would go with the izip from wall mart. I hear really good things about it. The only thing I don't like about it is the battery arrangement. It has a proprietary battery pack system.
How to begins
also take a look at the how to begins post it is a discription of the build step by step...
If I were just starting in the ebike area, I would go with the walmart bike and just adapt it to other things. However my pusher runs a non geared scooter motor. In theory it is a better performer. I cant swear to that though.