HD video capture with overlay? (Think dashcam stuff)

[Nogginboink]

I'm curious to know what it would take to develop an HD DVR similar to the dashcam units easily found on AliExpress. I'd like to build a device that would allow me to capture HD video, overlay character text on top of the video, and store an output .mpg file on an SD card.

I haven't had much luck finding any chips that would perform these functions. Then again, this entire industry appears to be walled off from hobbyists.

Does anyone have any idea what semiconductor parts might be used to build a solution, and how to get hold of these parts from hobbyist-friendly distributors?