Get yourself a Woosh Big Bear. It's a cheap, solid and powerful bike. Woosh support this forum too, so you won't get any problems with after sales support. At 18st, you need a bike with a strong motor and a big battery. Those cheap folding bikes will probably collapse under your weight and the tiny batteries won't get you very far, as you'll most likely be using maximum power all the time. The quoted range is for a light person pedalling hard on a flat road. A heavy person on a hilly ride, would be lucky to get much more than 10 miles.