Thats similar to what im looking at , the t260 are the lowest payload arent they , about 650 litres would be fine i reckon, you will rarely run out of tht alaone
He will be running overweight full with an additional staff, gear and diesel.A 500litre tank in a Citroen Berlingo 800kg payload maxs out with full tank of diesel, kit and me as passenger.Spruce