A low powered pressure washer is more than enough power for car cleaning.
There's no point in getting a high powered one, unless you need the power for other purposes. The results will not be any better, or achieved any quicker and the risk of inflicting damage is much higher.
Whilst there's a bit of a stigma attached to pressure washers and the damage that they can do, if used sensibly, a low powered one will not inflict any damage, whereas a high powered one can cause significant damage!