It looks like I'll be renting a pressure washer. I also like the idea of the rental place maintaining it for me - so to speak.
Many rental places, the good ones, will offer advice and tips on how to use the tool effectively and may give you chance to try the tool out. This can be handy when they offer several different sizes of unit and you don't know which one to rent.
General advice is to rent a unit that is slightly larger and more powerful than what you think you can get by with. Most tools can be throttled back to work less aggressively but it is hard to get more power out of a small unit.
When you rent make sure that the agent clearly notes any dents, scratches or damage. Have them start up the machine and show you all the adjustments. Get the person's name clearly written on the rental papers so any understandings are honored. Most people are honest but better to have it in writing to prevent conflicts.
Make sure you are absolutely clear on delivery and return times. Observe them closely. Don't expect any breaks. Note that many rental outfits give you an extra day for free if you rent over a weekend and/or holiday and get it back to them early.
Ask about storage and theft. One outfit I used would give you a break on replacement cost if you used their chain and lock to secure the tool. They charge $5 extra. Use your own and you were on the hook for the full replacement cost. Some outfits may offer added insurance for a price. If you have job-site or homeowners insurance give your carrier a call to see if they cover it. Better to know about all this ahead of time.
IMHO renting equipment is a great option if you don't often use a tool. They store, secure, and maintain it for you. Industrial tools can get you into trouble if poorly used but they can increase safety and speed a job along, possibly eliminate the need to hire a contractor, if well used.
A friend rented a tow-able man-lift over a long weekend and used it to clean out his gutters, make repairs on his roof, scrub moss off his siding, and trim his trees. Working off the lift was easier, faster, and safer than working off an aluminum ladder. He clearly had some fun using what amounted to a carnival ride for adults.
It would have cost him several hundred dollars but with construction work dormant, the people who most commonly rent this sort of device, he was able to talk them into a half-cost deal. Neighbors saw him getting a lot done and rented units. The company made money off the deal.
Another option is to coordinate with neighbors. We rented a large towed industrial compressor and jackhammer and several people used it to knock out a number of small jobs quickly. If each person had used smaller electric jackhammers, or sledgehammers, it would have taken far longer and been much harder work for everyone. By combining jobs and user we could afford the more powerful tool.