Depending on how mechanically inclined you are
For a single cylinder 4-stroke engine you may want to have on hand an extra fuel tank, fuel filter, and fuel line. A valve kit and rotating nozzle repair kit may be worth having around too. Whatever model pump you use, you may want to buy a general repair kit for that model. These kits usually cost in the $20 to $50 range and can be very handy for common repairs. Plunger, valve, piston, and regulator repair kits are available too, with valve and plunger repair kits costing the most (in the $200 to $300 range). As for high pressure hoses, you'll almost certainly want to have extras on hand if pressure washing is an everyday part of your work. Have high pressure hoses available that are pressure rated for at or above the pressure your pump works at. Some people suggest that in situations where you require long hoses (say 100-foot or so), you should connect two hoses of half that length together. That way you can often replace a shorter length of hose, or in some situations you may be able to do part of the work with only the shorter hose until you can get another to connect to it rather than being completely out of luck if a longer hose gives out. Trigger spray guns are another consumable, though higher-end models are made to last longer and may come with repair kits so you can get the longest service possible from them. Having a variety of trigger spray guns available to suit the job can help you avoid down time as well. If some of your applications require the use of bleach, choose trigger spray guns designed to handle bleach and reserve them for your bleach applications. Having couplings and shut-off sockets available to accommodate your various systems is a good idea as well. Filters and strainers are smart to have around too, in order to keep your system from damage from particles that get in through your water supply. Even a little bit of sand can damage pumps and coils, and having a filter or scale removal system will protect your systems and prevent repairs. If you use hot water systems, then having an extra igniter and fuel pump for your hot water burner can save time if you have a breakdown. Miscellaneous items like thread locker, hose clamps, and pump oil are other items that should go in your general pressure washer tool kit.