i had a astra van with 250ltr flat tank, it was good little van with know probs what so ever. if you know what you are looking for when buying one , you soon learn to spot the bad ones even the tucked up ones, i would only ever look to buy a van with two owners and full history with low miles, not a cheap one with high miles or clocked,
as for banging in your head on the tail gate this is due to either being 7ft tall or carrying long ladders on top. now most of us still have a use for a ladder or carry one just in case, but if you change your ladder for a smaller triple it will solve the problem of banging in your head,
then if you find a 250ltr tank not enough its not hard to put a few pre filters in and a champ 4021 ro, this will give you the added bonus to fill up on your round.
the new astra vans have a better payload than older models, i would have a astra van set up any day great little vans.