I just jumped in to the whole thing, didn't really research it much. I originally was going to use a ICE engine on my Xtracycle build, (again, just jumped in) I decided that an ICE wouldn't work for me. As I hung out more on the Xtracycle board, I heard about e-assist, decided that would be an option. I checked out Cycle 9. They have a (well deserved) good rep on the Xtracycle board.

I bought a complete kit from them, so that I knew everything would play nice with everything else. I went with a front hub, because I was/am playing with the idea of either an IGH, or a NuVinci rear hub.(someday)

Whitecat is absolutely right: The battery is the most expensive/important part, not a place to cheap out. The only point I disagree on is about front hubs being finicky to ride with.

The endless sphere forum sounds like a good resource. I have no experience with them.