There is nothing too wrong with your initial idea, many start out in a similar way and gradually mutate their systems as time passes until you arrive at what you personally find is ideal for your particular circumstances.
Although I have a full van mount system, I also have the WCW backpack...which was long ago converted to a sacktruck trolley mount system.
I did away with the little battery on it, got a small car battery, a cheap, lightweight sacktuck and sat the backpack on top of the battery, all strapped on with bungee straps.
Might only have a 15/18 litre capacity but it is more than enough to do a single, standard semi for most WFP window cleaners (I can actually do 2 per re-fill if I have to, that's 12 windows per semi by the way).
To begin with, as you are learning, you will use a shed load of water but as time goes by you will hone your technique.
As for the continually re-refilling; just drop off a few barrels of water before you start, space them out right and it won't mean tiresome, continual re-trips to the van.
If you do decide on a van mount, a flat tank doesn't mean you can't use your van for any other purpose, just dump out the reel, barrels, backpack, hoses etc plenty of room on top of the flat tank after all.
Keep asking questions and every now and then a reply will appear that you will feel you can make use of, loads of us out there and we're all different!!
Ian.