if you're keeping your speed down to 10-12 mph, then 1000 watts of battery power should be enough with a small to mid size motor. 200-500 watts.
Unless you're a mail carrier who's going to be using this to accelerate the 30 meters between mailboxes for 80km. in that case, you're going to need a much bigger battery.
I'd recomend a geared hub motor like the low rpm version of the Puma.