I have a Peugeot Expert with 500l tank.. Fantastic van but wouldn't have another one for WFP for the following reasons:-
Fuel consumption terrible on such a small van ( I have the basic non HDI version). The HDI is a lot more miserly and quicker.
Most importantly the Payload on the basic version is easily exceeded. Full tank of water, actual plastic tank, metal cage, RO system etc, full tank of diesel, roofrack, ladder, driver,dog, pressure washer, trad gear,wfp poles, gutter lance other odds and sods, let alone with a passenger.
You should be ok driving on local town roads etc but hit a motorway,main A road etc, where there are some clued up Traffic Police,on your way to the first job of the day and you are risking a massive fine for overloading, probably £600 to £1000.
I went to a weighbridge, just out of interest, it cost me £10 and with the above kit in, full tanks and no passenger I was over the limit. Can't remember by how much but something like 40kgs, not major, but a passenger would have made it well over.