I thought I'd share my experience with this... I've had a Nexus 7 running in the dash of my car for about 6 months now. It's a pretty good solution as it does GPS, reversing camera, media playing, engine monitoring/diagnostics etc, has a much better screen resolution than any android based headunit that I've seen and runs more recent OS.
There was a lot of trial and error in getting everything to work... The nexus 7 will fit in most double DIN sized headunit holes.. In my case I have a built in headunit plus a double DIN hole where I removed the factory Japanese Nav and installed the Nexus. Looks factory. I can post some pictures if anyone's interested.
Parts required were:
Nexus 7 (Rooted, purpose built kernel for OTG mode while charging and some other awesome features) ~$225 Amazon & ~$25 for the kernel from the developer
High current 5v power supply ~$10 Aliexpress
Various USB cables/adapters/OTG/hub ~$25 Aliexpress
USB video grabber for reversing camera and associated app ~$15 Aliexpress
A few relays ~$10 Aliexpress
Audio Integration with factory stereo (aux mode) $ will vary greatly based on vehicle
Has anyone else tried a system like this?