I started with a 2400 PSI 2.0 GPM machine that I kept in my van. Mind you I am primarily a window cleaning contractor but I had a few customers that wanted small areas cleaned. I did one whole house that was DryVit and paid for my machine. I continued making money with that little gadget all summer just by doing small areas like patios and lawn furniture and front porches.
This spring I purchased a cold water unit at 3700/4GPM, a 5X8 trailer, 275 Gl. water tank, X-Jet, turbo nozzle, water broom, 350 ft. garden hose and various accessories. Total cost = $1,975.00 My first job this year was for a window cleaning customer who wanted her pool house, furniture, concrete patio and horse corrals cleaned. the white vinyl fencing was the really big part of the job. Almost a mile of fence to be cleaned inside and out. I made 1,800 on that job. The second job this summer was a metal building and parking lot that I made almost $2,000 on. The point is, if I ran the whole rig over a cliff tomorrow, I haven't lost a penny.
Mind you, I am already an established and respected window cleaning company, but I think you could have similar results. I am by no means an expert but I read these forums extensively and stick to professionally manufactured chems and follow the instructions meticulously. I also am very cautious not to get in over my head. If I think a situation is questionable, I refer the customer to a "big dog" power washer in the area. That way I am just skimming the cream off the top. So go for it, but be careful, you can do it.
Steve