Don't buy the cheapest of the cheap GoPro copies. They'll record to MJPG AVI files or other such rubbish formats and look like garbage.
If it is the trip of a lifetime, why don't you push the boat out and get a 360 camera (if you have the computer and also memory cards to cope with the file size). You can then change the viewing angle at any point, where a helmet camera is just going to be where you're looking.
If going for a Gopro style camera, the ones that fit into waterproof cases (rather than the camera being waterproof) generally won't record audio whilst in the case. Just be mindful of that, plus, if you plug in a charging lead, you may open a door meaning the camera then isn't waterproof at all.
The Drift Ghost XL does have this covered, has a waterproof charging port/cable, and can record audio. The footage is fine, but not at 4k Gopro level.