Do you have something like this in mind?
http://www.ebay.co.uk/itm/RENAULT-TRAFIC-VIVARO-CREW-CAB-CAMPER-VAN-CONVERSION-KIT-SEATS-BULKHEAD-/191109199077?pt=UK_Campervan_Caravan_Accessories&hash=item2c7efef4e5#ht_396wt_1161I would imagine a 250 litre tank across the back behind the bulkhead partition will be ok. You have been even able to go to a larger tank, but this would depend a exactly where the tank sits in the cargo area in relation to the front and rear axle.
I would contact Vauxhall technical dept as they will be able to provide you with a loading weight footprint of the model van you are considering, taking into account the payload of the van in question. Once you have that, you can then make an informed decision on the size of tank you could fit.
If you are just thinking of fitting rear seats without a dividing bulkhead then it would be easier to carry poles as they would slide under the seats.
If you are considering this then it will be far more comfortable for rear seat comfort to have the rear bulkhead, internal roof lining and side upholstry, especially in winter.
One of the issues you will have to contend with is water spills will make the internals damp and won't smell very pleasant. I would consider the rear behind the seats to be fiberglassed into a tray with a couple of drains to the outside to reduce this.
There is a local window cleaner who bought an old LWB Transit minibus, removed the rear seats and fitted a tank and works from that. Not the best looking vehicle but the main thing is that it works for him and his family.
However, I'm inclined to go with Dazmond's opinion on this. I would hate to have to go anywhere with a full tank of water on board because I hadn't used it - usually weather related. On the other hand on those odd occassions you do get caught, it isn't a big deal to have a seperate tap to drain it off in the street.
There is an old boy whose has an old smiley face Transit SWB for his business. It isn't window cleaning related, but whenever they go out together, whether grocery shopping or for an evening out, they prefer to take the van. Her car just sits in the drive and hardly ever gets used. They both find it easier to get in and out of the van.