I have already design the chassis and the body. Recharge battery during braking or downhill. 50 MPH this can be achieved in a small EV like yours with about 1000 watts, this is about 1.5 HP, adjustable speed can be achieved by using a speed controller. A speed controller is a way for delivering a variable voltage to the motor, it can also have a way to limit the max current and prevent a thermal motor burn out, this depends on the features.
Motor controllers are a commercially available electronic device.
This is the only way to provide an increase in torque at low speeds as when hill climbing or starting from a standstill. A 3 speed controller should be fine. The exact calculation of gear ratio depends on the type of use and terrain. Motor amps and volts and battery. A lead acid battery is about your only choice because of cost.