HI Clear view,
Big Money - the max tank i would be prepaired to install would be a 650lt baffled tank, only beacuse of the safety aspect.
You could approach one of the main manufacturers for a bigger tank, ie IOnics have quoted me in the past £1,000. for a 1000lt baffled tank in there frames (i think fitting was extra though).
The hose and hose reels would be down to want you want and i would recomend shopping around for these, 2 hose reals both with a 100 meters of hose could set you back as much as £300.
You could also try omni pole, as Glyn does some really good equipment.
For your kind of setup, basically i would recomend to buy the van without any set up, then add your own components, that will be the cheapest way forward for you.
Many Thanks
Andrew