I have three pressure washers (edit: four) (edit #2: five) with 13 - 15 HP engines putting out 4000 psi at 4 - 5 gallons per minute. I've bought them off of craigslist for fractions of a penny on the dollar. Direct drive and belt driven pumps, General, AR, Comet, and CAT pumps. In addition to the unloader, the packing kits sometimes need to be re-seated, and there are inlet and outlet valves for each piston which need to be cleaned.
Clean inlet water is a must: get a good inlet filter from your local pressure washer supply house, not just the little mesh screen that fits in the water hose. You can get a great filter for less than ten dollars.
Chemical injection after the pump is best. Many people think that high pressure is good, but honestly anyone that makes a comfortable living off of pressure washing will tell you that Gallons Per Minute is much more important. Hitting a house with 4000 PSI is asking for trouble, plus you have to be really close to do it, and cleaning the side of a house would take for ever if you keep the tip inches away from the cleaning surface.
Use large orifice tips to reduce pressure and create a fatter stream of water to shoot much higher. For my pressure washers, a #30 tip will open up the chemical injector, but a #25 tip won't, so I use a #30 for applying chemicals, and a #25 for high rinsing. My #25 tip is actually just a 1/8" hose barb screwed onto a NPT fitting that fits my pressure washer wand. With these two I can soap and rinse even some three story houses without my feet ever leaving the ground.
I'm a novice woodworker, but pretty knowledgeable about pressure washing if anyone ever needs advice, etc