I haven't read through much of this thread, no doubt some sound replies already.
If I am repeating, frgive me.
Do not send a new van on this type of work, often you cant get near so will nead 100 metres plus of HP hose. If you can get near, it will not be where you want a new van to be.
For a tank worth having you'll need 1000 litre. Then, you'll still need a constant feed, which will forever flood your van. Your van will also need to be left open, unless you go for a flatbed, which would work well, but then any chance of verstility is lost as it's no good for owt else.
I have found the best solution to be a bowser, tricky to manoevre around site until you get the hang of it, perfect once you have. You can drop it off if space is tight, no floods as overflow just runs down the side.
Definitely get a deisel PW, it will be running all day every day and deisel will save you about £14 a day.
As for subbing this out, I have also done this, rates are rarely what you would hope for and it generally ends badly. I tend to 'introduce' masonry cleaners as and when possible and agree a commision on all works undertaken for that customer. Cuts out the hassle for me and puts the masonry guy in direct contact to build more work, across regions quite often.