I would strongly suggest you get started with 4S, otherwise as a beginner you're just going to break a lot of stuff and will spend a lot of time and money repairing it. On 4S it is still plenty powerful and lots of fun.
As far as hosing it down when it gets dirty goes, it can be done for sure, but it's not as simple as hose it down and let it dry and you're finished. No RC car is completely waterproof. Let's assume the electronics are fully waterproof for example, that's great because you can wash it without having to take the electronics off, but some mechanical components will get damaged eventually by water getting in. In particular the bearings. So if you do hose it off to clean it, you then need to make sure you dry it properly, you need to make sure you don't hit it directly with the water jet at the bearings or diff cups or electronics, you'd be more like sprinkling it over and going over it with a brush then rinsing it all off. Also, every time I do this I follow with a partial rebuild to make sure metal parts don't rust, at a minimum I take off all the bearings dry them and oil them, open up the diff boxes to let them dry in case some water went in.
All of this can get tiresome and time consuming so these days I tend to avoid running in wet/muddy conditions. And one of the best tools I've bought for my RC hobby is an air compressor I have in the shed, whenever I get back from a session I just blow all the dirt off, some times the muck is a bit stuck in so I go over it with a paint brush or similar to release it and then blow it all off.