For 250 pounds weight, you will need some real wattage to get up hills. This is where the weight matters, uphill. On the pure flat, a smaller scoots motor can do it. Obviously, the frame itself needs to be adequate, meaning an adult design, not one made for 100 pound pre teens.
But if you have hills to climb, you need to consider something with a lot more power, larger hub motor, and perhaps 1000w.
Same thing goes for range, if they say 20 miles, figure on 10 being reality. When they say 20 miles, its been done with a light rider, no wind, no hills, and riding very very slow. Its not a lie, but its not 20 miles going fast.