650l easy.
really, I think not! you would be overloaded thats nearly 700 kg just the tank and water, thats without the tank frame, hose reels weigh about 20kg each
the maths roughly
water and tank 700kg
frame 30kg
2x hose reels 40kg
fuel 50kg (thats not full)
2 x men even if the skinnist in the world 100kg i weigh about 90kg
these figures dont include everything else we tend to carry and is all ready over the payload,
I would say 500L you could safely use and add a couple of barrels as and when needed