Waffle,
Hi i saw your post, I have been an Ebiker for over ten years and have alot of user experience with them. First off the battery is the most expensive and important part of the Ebike. Be very careful buying a battery from anyone not recommended, you will spend a Ton of money on something possibly useless.
I have a few recommendations for you.
I found barely used Laced direct drive hub motors for 150- no controller but great deal.
http://endless-sphere.com/forums/viewtopic.php?f=31&t=67047then batteries. the absolute cheapest is hobby king, theses are hobby batteries and can be dangerous, some learning is required, and you should charge and store where you could have a fire, like garage or fireplace. The chargers are expensive too.
THE better batteries come from EM3EV expensive but worth it.
then there is ping battery.com trusted seller.
or ebikes.ca ( aka grin tech)
You could get lucky and get a cheap and good battery from alibaba or ebay, however I think the chance you are taking is 50/50 . care to gamble with 500$ ?
I recommend the following forum, search under batteries, I personally lost over $1200 on bad batteries from china.
http://endless-sphere.com/forums/index.phpLastly If I were you I would look into a geared motor to start as these feel more bike like than the heavy direct drive motors.
good luck and AMA