What else would I need though bearing in mind I would be buying water purified already?
They're good backpacks, particularly for the money.
You'll need a pole of course, pole hose, connectors, and a brush. If you're only doing 1st story windows and/or conservatory roofs and 18ft (extended) would be OK for most and will give you 22ft reach. When comparing poles look for the extended length and the reach, as it may not always be clear from the model no.
Depending on how/where you're getting your water you might also want to get a TDS Meter (Total Dissolved Solids) and some 25L water containers.
I'd recommend something to control your water use as your supply will be quite limited (e.g. inline tap, trigger, Aqua-dapter), but if you're introducing it gradually into your existing traditional round, that might not be too much of a concern to begin with?
Paul