You can try something like this on the low end (I don't know about the quality of the bike but the Q100 and lithium battery are a good start)
http://www.bmsbattery.com/ebike/508-...c-bicycle.html
I know it says $500, but shipping is probably going to be a couple of hundred.


It depends a bit on how far and how fast you want to go, but it doesn't sound like you need a big expensive system.