For you guys that drive new custom what payloads and engine do you have I am test driving Lwb with 155 engine on the 330 model payload 1359 do you think this is overkill for 700 litres daily or could I get away with 290 or 310 models
It would be easier to pass comment if you gave a little more info: Single operator or two? Would you be carrying any heavy gear apart from your water tank e.g. powerwash kit/generator?
For WFP only as a single operator with a 700 litre tank, 1359kg is more payload than you need. However, maybe it's not such a bad thing to have some spare. I'm not so sure that constantly pushing vans to the limit is a good idea if you wish to keep them for many years. I usually reckon on a payload of at least 500kg above the weight of the water. I know that many operate on lower margins by being not as fat as me, not filling the fuel tank, not carrying much other kit - and that can be done legally. Take a passenger when your tank is full and things can get a bit tighter.
I had a van with 1200kg payload and an 800 litre tank. Filling the tank while full with diesel and carrying other kit would have put me slightly over (I paid to check on a weighbridge). N.B. I could do with losing 25-30kg in body weight.