What do you think about mid-drive motors? I've been considering one of these:
New Bafang BBS02 750W mid drive for electric bicycles | ELECTRICBIKE.COM
Mid drive, high power (up to 1000w if you take away the power limitations), and helical-cut gears so it's nice and quiet. Plus you can shift gears with it, so it seems like it'd be a bit more efficient.
Sam