I would really recommend the 'black' versions of a GoPro. The key reason is that the black versions include a surprisingly good GPS, and accelerometers. The Hero5 Black I have is rated at 18hz, and is much better than my phone GPS. This data can be read for accurate speed readings by video apps like RaceRender. With the data integrated in the video file, it is super simple for RaceRender to automatically pull it out and use the GPS data in the video.
You can see the speed readings in this video:
You don't need the latest GoPro. This video was done on a Hero5, which you can still find on close-out cheap. The Hero6 is starting to close out relatively cheap also. I also know some people who have good experience with a Garmin Virb.
Don't forget to budget for multiple high-quality micro SD cards for the camera. At 1440 res I get just over 2 hours of video on 64GB card, which just covers the typical 4x 25m sessions of a typical HPDE day for me. YMMV, and higher res eats more space.
I use a head-rest clamp for my camera. If you attach it to an occupied seat they are generally steady. I like to be able to see my steering inputs, and catch a glimpse of the gauges. Also really easy to power the cam from back power jack with a longer USB-C cable so don't have to worry about battery life. And easy to operate by just reaching back, and viewing it in the rear view mirror. This is the mount I use:
https://www.amazon.com/Headrest-Tack.../dp/B00XIB6SK4
Still, I have been thinking about moving to a windshield mount. It definitely gives a better view of the track. Harder to power though, and mounts can be trickier. Some groups also have issues with just using a suction mount for a camera. They want it mechanically secured.