go to endless sphere forum, there are tons of information about electric bikes.
Endless-sphere.com • Index page
Actually direct drive hub motor is not so efficient - if you run it lower speed and uphill, it draws much more current and your range suffers. Electric motor is most efficient when it runs its rated RPMs. So in real life geared hub can give you more range it you are not running full speed all time - and then you have wind resistance...
So most efficient would be direct drive hub with max speed under 20mph but then you cannot have high speed. If you pedal faster than hub motor max speed then it actually drags you down - it starts to regenerate. Lots of compromises.
So gears are good for efficiency - to keep motor at higher RPMs. And you get climbing ability and better top speed.
Check GNG mid-drive kit. I would go for that. Actually planning to buy one myself
Endless-sphere.com • View topic - GNG, 1000W 48V BB-drive, $400
Meelis