Depends
It depends on what you use it for. If you are only going to use it for washing the siding on your house or a deck,anything over 1500 psi should do. If you have grease,oil or concrete to clean. It would be cheaper to rent a hot water type 2000 to 3500psi unit.
We run a Hotsy capable of steam and 3800psi. It definitly does the job on grease and oil. You have to be careful with it however,it will strip paint. It will also clean decks very well,however do not forget to route pressure hose on concrete and not across the lawn. (the later I did and could not figure out why I had a dead strip across lawn)... ...
Things to stay away from are vertical pumps,comercial grade soaps,Non-biodegradable,and sprayers you hook to your garden hose claiming to clean anything...