Hi, I am not in a position to tell you which to buy but I would go for the largest tank your current van/vech can fit, that is not to say you have to fill it to the brim.
I initially bought a 250l tank and thought it would be more than enough, within days it was noticable that was not the case....go large on the tank bearing on mind the load capacity of your vech and all your other kit.
Think about microbore(6mm id) or minibore (8mm id) hose.
Your circumstance will dictate what is best poles, pump/controller,water production system for you to buy...why not aks to see if a wfp w/c in your area will let you tag along for the day....it may well give you some idea's.
As with all things you can spend ridiculus amounts of money on the all singing all dancing set-up and then again you could build your own set-up for a fraction of the price...lets face it were cleaning windows

I am sure a few other members here will be along shortly and will be better qualified than I to give you detailed advice.
Good luck Dave.