I have converted a vw bus to 112 VDC electric drive (Tesla battery modules 22.5VDC X 5 ea). There is also a 12V system for lights and instruments. The BMS supports SPI and CAN. I have no experience with either of these protocols but I am using SPI for the BMS. I have also heard of Android Automotive. I want to add a Tesla style tablet display, cameras, and proximity sensors and have expandibility. What are the pros and cons of vehicle network protocols? Can you point me to examples to learn from?
You would have to find the most efficient way to integrate a dependable 12V rail into the car, which probably means a discrete 12V battery and BMS, charged by a DC-DC converter. All the equipment is standard 12V or 5V.
Search google for "12 inch tablet with sim card slot" or whatever size and features you want. If no sim, then you just have to pair it with bluetooth. Tesla has a 12.3" screen so 12 will be fine.