Speaking of damage, I would look into insurance for this operation. A power washer can do a lot of damage, especially if used on the house. Even driveways can be ruined by power washing, as it can blast the finer cement out. Decks are the same thing, they look great when wet, but the water...
There was a guy handing out flyers in my friend's neighborhood offering to power wash people's driveways. My friend decided to ask for a quote, but this fellow's first question was whether he could borrow my friend's pressure washer to do the job! Needless to say, that didn't go far. No idea...