Tony,
the most important figure you are looking for is payload - the rest is irrelevant IMHO. Payload is what the van will carry including tools, accessories (roofrack/roof bars, bulkhead,) driver and any passengers, and fuel.
Lets see what you want.
500 litre baffelled tank - 35kgs
When full the water weighs 485kgs. According to Wyedale the tank manufacturers the baffels reduce the volume of water the tank holds.
Tank frame - 15kgs
Hose reel with 100 meters minibore full of water - 30kgs.
Ladder - 10kgs
Roof bars - 15kgs.
Leisure battery, pump. pole and accessories 25kgs.
Yourself - 100kgs
Tank of diesel - 80kgs
Towbar - 20kgs (if fitted)
If all those weights are correct then this comes to 815kgs.
So as a single operator you could get away with a LWB Transit Connect L as it has a payload of 825kgs. But the moment you add additional equipment/people then the figures change.
500 litre baffelled tank - 35kgs
When full the water weighs 485kgs.
Tank frame - 15kgs
2 x Hose reels with 100 meters minibore full of water - 60kgs.
Ladder - 10kgs
Roof bars - 15kgs.
Leisure battery, pumps. poles and accessories 35kgs.
Yourself plus your mate - 200kgs
Tank of diesel - 80kgs
Towbar - 20kgs (if fitted)
Again, if these weights are correct then the total weight carried is 955kgs.
In the second situation you need to be looking at a Vivaro or equivalent with a payload of about 1200kgs.
I have a Citroen Relay SWB, payload 1154kgs with a 650 litre tank, 2 man operation. This is fullly laden on the odd occassion when 2 of us go out together.
My son has a 52 plate Peugeot Partner 800LX (800kg payload). We have fitted a 500 litre tank, single operator. With a full tank and him the vehicle is OK weightwise (payload). If I get in, then his van will be overloaded if he has a full tank of diesel.