Paul
A little bit of a mixed message there.
You really don't want to be plugging a pressure washer into a commercial customers electricity supply and asking them to use their water.
Commercial clients, in my opinion, expect a bowser minimum and a petrol/diesel pressure washer, a self contained unit.
Even domestic I would be loathe to ask them if I can plug into their electric.
Petrol will always be more expensive than diesel (you can use red diesel) and the running costs will be about a tenner per day on petrol & £4.00 per day on red.
If you want to knock on the door of commercial clients, at some point (no need to be buying two machines) then my recommendation would be a 15lpm (minimum) machine, diesel with a minimum 3000psi.
These specs will allow you to offer a superb domestic service and will clean virtually any domestic challenge and will give you the confidence to actually knock on commercial clients doors.
There are a lot of machines always available on Ebay, Hilta, Honda, Lombardini & Hatz.
Keep away from the chinese stuff, may work for a while but when you need a part where are you going to get it from?
Get a service guy to maintain the machine. Make sure it has a return to tank, most pumps are water cooled and you need this to stop the pump from going bang.
People will give you plenty of advice on here, if you see a machine for sale post it on here and let people comment
Rob