green stuff is easier removed if you treat it first - I use a product called Chlorus (think that is the spelling) - get it in local co-op shops - dilute per instructions, pour on affected areas, quick brush & you can use ordinary garden hose to clean down - all depends on how bad the green stuff is - assuming its not moss, just a general green covering on the concrete?
I have a pressure washer - but nothing as powerful as the one you are looking into - picked it up in Aldi a few years back - electric one, but does the job...