driver - 80kgs
fuel - 50kgs
tank - 30kgs
water - 400kgs
battery and other fittings - 30kgs
Total 590kgs
This is allowing for a full tank of water and carrying all your extras, if you feel you will be carrying more than that and or more than your vans payload then either change van or reduce the amount of water you will carry.
I've done plenty of 600kg payload vans with 400ltr systems and there's no problem driving with a full load, its still quite safe, plus if you opt for the 400ltr baffled tank then remember its not actually a 400ltr tank anymore as the baffles take up some of the volume (about 50ltrs) so you're only carrying 350ltrs and thats if you fill it to the throat. I only noticed this a few days ago when filling a 400ltr baffled tank with 25ltr drums, it only took 18 of them and was overflowing.