if it is just for Auto detail or basic Car wash, any 4hp / 5hp
1200psi / 2500Psi is enough Power & Psi
you won´t be using over 1500Psi for cleaning cars, over 1500psi is too much!!
the Brand counts for sure, any Honda engine, B&Stratton,De-walt,Ridgid will do the job
from electric to a Gas engine, Gas engine is more heavy duty use, more Noise but sure more power & reliable than electric.
check The Northstar tools / they have great deals!!