Hi Teri,
If the batteries (12V 75AH x 2) can supply 1800 WH and that is supposed to allow 25 miles at 10 mph, thats about 2.5 hours or about 720 Watts continuous.
Maybe that is for 500 lbs. total weight. That implies about 7% rolling resistance. A grade of 7% will double the load on the motor. And the batteries will drain in half the time. But that will require more power than the motor can deliver so you will slow down. 6% grade is a steep hill on the interstate. The steepest street in Los Angels is 33% grade (which takes a hero to pedal up). So between those maybe you can guess how steep of a grade you will need to climb.
If all this is close to the right numbers and you want to glide up 10% grades on battery power you will need 2.5 HP or 1760 Watts. If you want to extend your range you'll need more batteries. Both also add to the weight.
Just to give you some idea of costs, motors cost between $500 and $1000 a controller around $500 and somebody to do the mechanical work.
-mort