Total combined weight, plan for at least 400-500lbs (181-226 kg). If you have a cycle-truck or a velomobile that can hold two people, think about at least 600-800lbs (272-362 kg).
a.) Find a power calculator or find a graph for your motor, like the bicycle speed calculator at kreuzotter.de that has a setting for Velomobiles. http://www.kreuzotter.de/english/espeed.htm
b.) Then a gear ratio calculator, like the motor and gear ratio calculator at Electric Scooter Parts Support Center.
c.) Take the RPM speed that your motor will produce at the wattage you will need (see a performance graph of the motor). Calculate a gear reduction ratio you will need to reduce the speed of the motor to get the maximum thrust. Subtract a little speed (1-2 mph or 1.6-3.2 kph) for the lack of accuracy.
Find the peak efficiency R.P.M.s and wattage on the label. When a motor is pushed past the peak efficiency limits too long, it can over heat. Brushless motors can produce about twice the power on their label for a few minutes without over heating too much; to get past the steepest part of the hill.
Something else to consider is that if your motor nameplate says 1200W, that's the electric power it draws. Its actual output will be less, possibly considerably less depending on the motor design. Yet brushless motors in general can use twice the nominal wattage.
In normal mode, or the constant torque region, the motor exerts constant torque (rated torque) over the entire speed range until the rated speed is reached. Once past the rated RPM speed of the motor, the torque will decrease proportionally with speed, resulting in a constant power (rated power) output. The constant power region eventually degrades at high speeds, in which the torque decreases proportionally with the square of the speed.