finding the right torque and motors
Energy = 0.5mv^2 (I'll use SI units here) so to reach 20km/h = 5.55m/s you'll need to provide 0.5 * 1500N * 5.55^2 = 23kJ.  If you want to achieve that in 10 seconds you'll need 2.3kW output.  Intuitively that makes sense, as a 2HP moped will return about that level of performance with slightly less mass and power.  A 10cm diameter wheel has a circumference of 31.4cm, so at the midpoint of the acceleration 2.775m/s it will be rotating at 8.84 revolutions per second = 530rpm.  That's irrelevant but interesting.  The propulsive force when accelerating is given by F=ma where m is 150kg and a is 0.555m/s^2 so you'll need 82N of force.  With a wheel radius of 0.05m that's 4.16Nm of torque.