Some good stuff here guys.
An idea - put a 400L in the van (if it's got the 110HDi engine it will be fine) you have - perfect for weight and 2 men and gear. Have a "cheap" 400kg trailer with a 250L tank on board that you can use behind it and other vehicles when necessary.
Hi Gold,
That is also another option. Ian has a 1.9d (815kg Dispatch - 90% of the vans Citroen sold were 1.9d's. There were a few 815kg TD's in the very early days, but they then upgraded the 1.9TD to a 900kg van, staying with that with the introduction of the Hdi engine and through the 'facelift' in 2003/4) but that will be ok for towing as well, albeit a little slow.
If he goes down the route of using his customer's water supply to supplement his daily requirements, then the most important consideration is going to be where those taps are that he can use. If the water supply is at an inconvenient place, then he may have to consider a small trailer, or even bigger trolley systems, such as a 125l tank on a 4 wheeled gardiners type trolley that can be transported to site in the back of the van and manually pulled around where needed. We also don't know what his site is (guess it could be a hospital

) and if so are there any external windows that can only be got at going through the main buildings?
There are so many variables that only he can answer - and many he will only find out about once he starts the contract.