Verkon S is an excellent disinfectant , it's used all over the place in disease control as well
, it turns pink in water
I've run hundreds & hundreds of litres of it through the dosing tank on my Nilfisk pressure washers over the years .
Cost is always the limiting factor but going for the cheapest is a false economy as a good washer will usually outlast three or more cheap ones .
Drain them down and after every use and always store them awy in a frost fre place as a good freeze often slits the casings/body of the pump unit .
If you don't have your washer yet , think of buying one that has a reinforced rubber hose rather than a plastic /nylon one as the reinforced rubber ones last a lot longer so long as you store them out of sunlight & heat and away from grease & oils .
I'd also recommend that you get a washer that has an extension hose on it as this often stops you twisting the hose around as you work , plus you don't have to keep moving the washer pump and incoming water supply hose around.
If you can get a washer with two changeable heads it's very worthwhile . One for a rotating spray cone which is good as a floor cleaner and the other as an adjustable wide blade to a point spray blaster that is good for channels .
Having the extension hose also means that if needs be you can add a drain blaster head to unblock rainwater down pipes & underground 100 mm dia pipe etc with ease for up to 30 mtrs or so . Any longer in the hose dept and the hose expands and contracts so much with the pulses of the pumping elements that it struggles to preform the best pressure spray form.
Re:- the barrel & header tank .
Its best to have some sort of in line filter on the incoming water if you are using rain water in a header barrel . Having the barrel three feet higher than the washer inlet is very helpful to ensure that the pump has enough head of water pressure when the level in the tank is low ..... To stop the pumping elements drawing in air and it causing wear.