I researched the same thing over the past six months. What I came up with was a ContourGPS camera, my existing Mychron dash, and Dashware software. The ContourGPS camera has built-in GPS that gives you accurate positioning, G-forces and speed. Dashware allows you to load the video file from the ContourGPS and it automatically extracts the GPS data. Dashware also allows you to create custom gauges easily to display the info. Lastly it pulls in the Mychron data for revs and laptimes.
I picked up a second hand, like-new, ContourGPS on eBay for $100.00. The Dashware software is only $50.00. So it was really an affordable package.