For the water purifying you will need a RO unit. 100gpd for £100
www.ro-man.com/shop/index.php/cPath/22_91DI resin from the same place is around £35 for a canister + resin
tds meter from anywhere for around £15
A water butt from bnq for £30 or something, to store the pure water.
25 litre barrels. Tenner each?
This place does a backpack for £80 ex vat. Out of stock at the moment
though.
www.windowcleaningwarehouse.com/The above comes to around £320. With 4 X 25 litre barrels.
And then you need to buy a pole. 24 ft pole would be best for gutter/fascia cleans. They are around £120 new.
An 18 ft pole would reach most average houses. And a bit cheaper.
So around £400 to get you started wfp. Another 50 quid on trad gear and your away.
You should look in the buy and sell section of this forum as you could find things cheaper in there.
I dunno if these are the cheapest places cos there are plenty of good suppliers out there.
Good luck.