I used a Suzuki Carry to pull my first wfp trailer. It now gets used for collections and to take the dog down to the beach.
I was going to fit 2 x 125l tanks that I have into it so the van could be used as a backup and for the wife to use. I put the 2 tanks in behind the back seats and filled them with water. The van setted down very low on it's suspension. So personally I wouldn't go any more than that.
The other problem is that it doesn't use much less fuel that my Citroen Relay 2.0 Hdi with a 650l tank.
I think Gold has correctly pointed out that 250l is not enough for a day's work WFP top and bottoms so you will need to top up during the day. As he states, you will probably outgrow it before you start.
You will be able to pull a trailer with a bigger tank, but anything above 250l will need a braked trailer. (I had a trailer with 3 x 125l tanks and needed to carry 100 litres in the van to act as ballast. With the trailer full it weighed more than half of the permissable weight of the tow vehicle for an unbraked trailer. On a good working day I often got through all my water including the ballast water. Smaller jets and flow controllers will have reduced water useage since then. On the same round - I will use about 350l now compared to 475l then.)
In my experience, this van is not a good van to use for WFP.
Spruce