I bought a Stihl RB 800 (their most powerful commercial model) and I am very happy with it.
It has the ability to put out enormous pressure - 4,200 PSI at 4 US GPM - but it comes with a pressure regulator to let you dial that pressure down. I normally run it at a reduced pressure, but occasionally I need full pressure for a particular problem such as paint on concrete, and it's great to have that higher pressure available.
I also suggest you buy a ripper nozzle (sometimes also called a rotating nozzle) - you won't need it all the time, but it is a huge time-saver when you do need it. Don't ever drop that rotating nozzle - they have a ceramic ball inside, if you drop it on a hard surface, the ball can shatter, and then the nozzle is junk. Treat it gently when it is off the wand.
I don't recommend Stihl's rotating surface washer - the one that
@gchidest recommended in post #8 of this discussion would be a much better choice. Just be sure it is rated to handle the output pressure of the RB 800.
Michael