Re: Many questions, wanna make a purchase!
If you keep your speed down to something like 20 mph, a walmart bike would last a while longer. You have to take into consideration it was designed with about 10-15 mph top speed in mind.
When you start going 40 mph, everything has to be heavy duty bicycle quality. Not much on a $100 Walmart bicycle can be considered heavy duty.
Another thing you might want to look into is legality. In Florida a china girl style motor bicycle should be considered legal, but it seems like the state authorities have some kind of vendetta out for them.