Hi Paul,
It would depend on how big of a window cleaning operation you have.
I am just small fry compared to some of the guys on here and just use a backpack. I purify my water at home with RO/DI into 3 x 200 Litre drums, then transfer into smaller 25 Litre drums for my car.
The RO I got from
http://www.johnhewitt.com/wc.htmBasically I just run a hose from your outside tap to the RO unit, send the pure to the 200 Litre drums and waste to drain.
John Hewitt will help you if you need advice.
Hope that helps a little,
Gaz