The specifications are:

Speed : 2m/s maximum
Weight : 150 max kg
Gearbox :step down gearbox (this is compulsory, so I don't have a choice using a speed controller)

I started off by calculating the torque requirements for this vehicle :

The forces are rolling resistance, acceleration and slope requirement.

Mass Force exerted on the wheels = 1500N

I assume the coefficient of rolling friction to be 0.1
Rolling resistance,Fr = Cr*N
= 0.1*1500
= 150N

Slope requirement,Fs = 1 on 5
= 1500*0.2
= 300N

Acceleration, 0 to 2 m/s in 2 seconds
F = m*a
=150* (2-0)/2
=150N

Total force required (worst possible case) =150+150+300
=600N

Total torque require at wheel end = 600*(wheel diameter,D)
= 600*150mm
=90Nm

Before I start the motor selection, I am using a Step Down gearbox with a maximum rpm input from the motor of 2000rpm(assuming 100% transmission efficiency). The gear ratio varies from 17:1 to 68:1. I find the wheel diameter which I have use for calculating torque by using the relationship :

Rpm = speed/phi*D

I know the output rpm and by selecting different diameter of wheel the range of speed would vary accordingly. I have found out that a 150mm diameter wheel would give out a maximum speed of about 2m/s which is the max specification speed.

I choose a bosch 24V 750Watt

https://www.boschmotorsandcontrols.co.uk/elektromotoren/produkt/0130302001/index.htm

Nominal voltage UN 24 V

Nominal power PN 750 W

Nominal current IN 40 A

Nominal speed nN 3300 min-1

Continuous torque MN 2,2 Nm

Breakaway torque MA 11 Nm

Direction of rotation L

Type of duty S 1

Degree of protection IP 10

Weight approx. 3,8 kg

Part number 0 130 302 001

The motor will be gear down about half so that it can be use for the Step Down gearbox which have a maximum input rpm requirement of 2000rpm(I realize that this will make the max speed lower than 2m/s but it will almost double the torque).

The total torque require to overcome is 90Nm.sice the breakaway/stall torque is 11Nm, the output torque is :

Motor torque =11*(gear ratio to Step Down gearbox)*(lowest gear
ratio of gearbox)
=11*2*17
=374Nm>90Nm

So I conclude that the motor is sufficient enough to perform the task.