Why not go with lithium packs? Kind of a lurker here... but I do have an electric bicycle I built using a road bike, brushless motor, and lithium packs. I'll try to create a thread for it somewhere.
Hobbyking has great deals on lithium packs.
For example:
Turnigy 4000mAh 4S 20C Lipo Pack (USA Warehouse)
4AH 4s, 20C discharge. Putting together 2 in parallel, 10 in series and you'll have a super light 40AH pack at 30+ volts for $633.
As for the price, you can do better. Here's a little known secret for hobbyking, find the best price point for the batteries, then wait on the page. If you have to, walk away for 5 minutes or so. A popup will eventually come up enticing you to buy with a lower price. Add to cart, then multiply the number of batteries in your cart so they're all at the low rate. Saved my big on my electrical bicycle. (I used a total of a 8S, 10AH pack)
