Well, that depends on you. If you made all your models to a size of 1 unit = 1m and use 1 unit = 1m as a distance measure, just like recommended in the documentation, and the standard gravity is 9.8 then it is logical that you continue using metric or better SI units all the way. The 9.8 would then be [m*s⁻²], a torque you apply in [Nm], and a force in [N].
Just be consistent throughout your whole game.
Edit: Ah, you mean the vehicle_body. Idk if they follow their own recommendation in there. They suggest to try a value between 25 and 50 for a vehicle with a mass of 1000. So, that does not sound like Watts, which would be the unit one expects. So far for consistency :-/
Actually, 25hp (around 19,000W, which is the unit one would expect) is pretty close to my first car that had a mass of 750kg and barely made 100km/h with a strong tail wind :-) But hps have more than one definition. So, I think, you just have to try, or start with the hypothesis that the unit expected is somewhat close to kW ...
