You'll have to check your local regulations regarding the recovery/dumping of the water if done commercially... A homeowner can let the water run to the storm drain, a business can't... My friends who do it on Military installations don't need to recover the water...
Check with your insurance company about what it will cost to get insurance to add pressure washing as a service. Pressure washing firms do a lot of damage by forcing water under siding and through badly sealed gaps.
Or... they get blamed for doing a lot of pre-existing damage.
I'd like to know more about it too...as I just went to look at a job for pressure washing a stucco fence wall around a condo community. It's probably more then a mile long and on average six feet tall.
I think I'll get estimates for myself from pressure washers to see if its even a good idea to go for it. Maybe just sub it out...