Thanks for the replies, guys. I'm pretty sure I want to build my own battery pack from scratch (36V Li-Ion or Li-Po). I'd like to buy a COTS front wheel w/motor (750W?) and COTS controller. I can do all the wiring. I don't need anything fancy like a display. I'd also like the ability to convert the bike back to non-electric in 15 minutes or so.

I took a look at the E+ bike kit - I don't like having the battery pack in the front wheel, I'd rather have it on top of a rear rack. But otherwise it looked pretty nice. I will do some more research and talk to Justin at ebikes. Thanks for all the help.