if you want to use a car, your best bet would be to fill up and purify your water at home and transport it in your car , then you can use a portable system such as a trolly or backpacks to clean your windows, in my opinion diy backpacks are more handy than trollys and a hell of a lot cheaper, check out diy wagga on youtube to see what i meen.
a backpack costs 140 including vat you can see them at
www.windowcleaningwarehouse.co.ukfor a pole you will want a clx 22 - 27 ft gardinerpolesystems.co.uk are the best by a mile for poles, trust me dont get a pole from anywhere else
you will need other bits a pieces like an ro system and a d i vessel and some resin, but the guys at the warehouse or gardiners can help you out with these they all know what you will need.
i recon for a complete backpack on a sackbarrow set up all in you will need about 500 quid
if you would prefer the trolly systems all in you will need about 1000 and the best place to find these are purefreeedom.co.uk