I've written a wiki article about sizing your battery pack
here, If i remember correctly I think the forkenswift gets about 200Wh/mile. I would go for as high a voltage as you can afford, higher voltage means less amps, higher efficiency, more power and higher top speed. I am sure Darin can testify as to the performance of a 48V motor, it might meet your needs but it doesn't do EV PR much good to have a car that slow. Its a bit more expensive but if you use golf cart parts and voltages you are just going to get a heavy golf cart.