hi im a newbie . i need too build a electric bike . advice please

only envelopes. so the extra weight is low . 90% flat lands . i have been reading many threads an i like the motor at the crank. or maybe 2 motors . each at the hubs .:)
I don't think you can get 50 miles out of an electric unless you can switch out your batteries a couple of times during the day.
At very low speed, a crank drive system is the way to go, electric motors don't like low RPM, with gearing you allow the motor to rev at 2000 rpm and crawl along slowly.

Still.. for real deal 80 kms per day, it wont' be cheap on the battery side.

options include

Carry a high-power charger, opertunity charge when you can during lunch and breaks...

If the route offers the option of a pit stop half way.. swap packs (2 packs.. double the cost.. but carry only one at a time to cut down on weight )

Pedal alot and use the motor sparingly..
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.