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Spend as much as you can afford if you plan to keep it for a long time. The large patio cleaning heads are great for keeping the overspray down but I think they need the larger models to power them.
Quality is a bit hit and miss until you get to the pro models. The domestic stuff has lots of crappy plastic parts that although replaceable are annoying. Also be careful about storing it above freezing.
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Mates got a middle aisle pressure washer and it cleaned up their patio perfectly, so Idk how fancy they actually need to be for occasional use.
I've got a baby karcher (k2?) on permaloan from my mum. It has the patio attachment which is fairly good Imo. If you're thoughtful about how you pack it down it doesn't take up too much space - which for me is important.
You can also easily get a range of attachments like foam guns as well as spares.
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I've got a Karcher K2 which is the cheapest model and ~ £60 I think. Plenty powerful enough to get rid of most muck and also risk washing out the grout.
I did end up buying a few extras (the patio attachment and variable wand) to make it more usable so you may be better getting a set with those in, although I generally got them by setting up price alerts and getting them when they were cheap.
pressure washers - talk to me. are the £60 bosch/karcher ones crap or do i need to be spending >£100. occasional use only for cleaning patio etc.