More or less agree with TINAS above. I use both a Hero4Black and a Hero11Mini when I am filming jumping (two different setups, one with 1080p/60fps and 4k/60fps for competition and the other for stills and 4k/60fps for tandems) and the only things that are really different between the two are stabilisation (GP11 is buttery smooth, 4Black is quite jittery) and colour depth (4Black is what I would call more natural, 11Mini is richer). 8s, 9s and 10s are all decent choices second hand though as they have Hypersmooth and decent resolutions.
Other than that, DJi Osmo are decent and Sony do one that is pretty good for 1080p and 4k. Not many of those on the second hand market though.
The GoPro suction mount is pretty solid but, as said above, as soon as you tack arms and stuff to the side, _any_ vibration will shake the picture. I used to film country road MC journeys with the 4Black and it got quite blurry. Have not tried it with the 11 yet, but I think it would be much better.