What GoPro camera should I get?

I was hoping to find a small, lightweight waterproof camera for my outdoor adventures, and after a quick search it seemed that a gopro was what I'm looking for, but after checking out the website, i see that the newer ones run a little expensive, but the older ones are slightly cheaper.
I was thinking of going for the older ones, but there's the newer ones with all the cool new features and I'm just wondering for people who know them better than I do which one I should get, or what I need.

I'm not going to be doing anything too intensive. Hiking, kayaking, underwater swimming (not diving), mountain biking, skiing, etc… Nothing too crazy. I've read some reviews and I don't think I need all the features in the newer ones, so I figure I could go a little cheaper with the older ones, but which one should I get?

Also, it would be nice if there was a screen on the back to see what the picture looked like.

Also, if I were to mount it at all, it would probably be to a kayak, a bike, a helmet or a little harness thing, if it matters.

Thanks:)

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