In a kit, at the bare minium, you need a motor, and a controller. And then there is the battery but you can buy that separately.
As for lithium, these are the cheapest lithium batteries on the market. I bought mine from here, also.
They require some extra maintenance and you have to know what you are doing a little bit in the beginning so you don't destroy them.
If you want an easy, plug, charge, and use lithium battery then there are other websites that sell them. If you want to know how to properly use the above battery see the thread on this forum titled "Looking for Battery recommendations"...there's a lot of good info in there that will help you use those batteries properly.
You can find some easy-of-use lithium batteries from pingbattery.com . They have a pretty good reputation but they ship out of china so you will pay $100 - $150 extra on top of the listed prices because of shipping.
Also NIMH or NICD. Ebikes.ca has a 36v8AH NICD for $240 + shipping
NICD doesn't last as long as lithium, though but it will get you about 14 - 16 miles though in range. If you want to go further than about 14 miles, then you'll need more capacity than 8 AH. I have a 20AH lithium battery from elitepowersolutions.com . I can go 20 miles or more on it but you might not ever need that much.
Last edited by morph999; 04-03-10 at 09:10 PM.