The vehicle power that you have found is the ideal power. It does not include any resistance forces that have not been taken into account. For example, air drag, heating (=friction) due to parts interacting, mechanical losses (things that bend / change shape during motion), etc.
To estimate the required motor power, the idea is to analyze the whole path from the motor shaft until where the main force occurs (tyre to ground traction). By combining (=multiply) the efficiency losses at each chain of force transfer, the general efficiency of the drive mechanism can be estimated.
The maximum power is needed when maximum force is required. Mainly the maximum speed for vehicles that aerodynamics dominate and maximum climb angle for vehicles like trucks.