macmac I have 1 set of 3m double wooden ladders, rhino triple roof bars and roller plus trad gear, plylined, and 600l water with me in van is right on the limit of payload (900kg). So in theory take of 300kg from payload of van to give max tank size.
Other factors will come into play such as weight of driver and tank (mine is heavier than most because it is 10mm thick polypropylene and honeycomb baffled - unlike brodex). Only way to be sure is to fill van and put it on a weighbridge. If you are a bit over just don't fill tank to the brim.
Ok so if a pauload was 915kg then using this formula the tank could be 615litres ..... or 650 not completely filled up.
That is gonna be the deal now as i am no longer getting the 1200kg payload vehicle but a 915kg one.
I have 2.0hdi dispatch (old shape). Would think you would be OK with 650l tank if not filled to brim. My personal experience is that this type of vehicle would carry 600l OK, but to be on safe side when you have it kitted out, fill it and take it to a weighbridge. I just gave the guy £10 cash as I did not require any official paperwork but was doing it for my peace of mind and to ensure that I was "legal".