I've been using a Hero 5 black mounted to my helmet for the last few years and it has been fantastic. If I were to buy one now I'd get at least an 8 though. I bought a custom mount for my helmet:
https://ridetechmoto.com/products/hjc-rp...chin-mount
You can also get generic gopro mounts on Amazon or make a custom one with JB Weld like Gone mentioned. The batteries for me last about an hour, so you will need extras on hand or you can buy a battery bank and plug the GoPro in as needed. While riding, I keep the battery bank in my chest pocket and plug the GoPro in if I'm going on a long ride. This makes it so I don't have to stop and change batteries. Also, I'd buy the largest size SD card the camera can support. You'll use up the storage quick.