I got a new Doblo nearly three years ago. 736kg payload.
400L tank in it. I sometimes carry an extra 50L in barrels to top up. It's fine, though when I replace I will get something with a 650L, two person operation possibility.
The old (pre 57plate) Scudo, Expert, Despatch are ideal (900kg payload I think) for what you require and you could put a 500 or even 650 tank in if you are very careful on other gear/passengers. Also more practical for family with three seat versions.