The Shurflo backpack is the best; though it's expensive compared to others. I've had mine about four years now, and it gets a right hammering and it's still going strong. Buy a spare battery if/when you get one.
Also the lightweight fishing trolleys are best to cart the thing about; something with pneumatic tyres.
If you work with your partner, you do all the upstairs and difficult to reach ground floor windows, including all conservatories; all leaded and Georgian windows with the backpack. And you partner should do all the easy to reach ground floor windows, knock for payment or leave envelopes.
When starting, think about your starting point so your partner isn't working first on a 'drippy window'.
When doing inside/outside jobs (we do a few offices/commercial units), I do all the outside with WFP and she starts on the inside.
With a backpack and some good team work you can do well with a backpack and rarely ever have to use ladders.