I think 5. Definitely 7
I’ve got the hero 5 black and have zero problems with it. The stabilization works well without completely deadening the video like some of the newer GoPros do. (I like to see some of the bumps and hops in the video)
Quality is good enough, it has GPS, and it’s only $100. Never had any problems with it apart from a broken mount.
I have a 10 and it works great! I would recommend using a cage or something that makes taking it on and off the kart easier between sessions. On hot days, if I leave it on the kart in the sun, it will overheat and stop recording <2 minutes into the session. So, I use a RAM mount with a B size ball to 1/4-20 bolt. I then use a 1/4" bolt to GoPro buckle adapter which I used permanent locktite on the screws. Now I can just quickly buckle and un-buckle the GoPro and put it in my pocket until the next session.