We could switch from a Raspberry Pi to an oDroid or another sort of low-power computer-on-a-board. My Pi has a task right now, so if I was to go forward with this, I'll have to get something new anyway, but for sake of this discussion, we'll assume this is it.
I own a USB GPS unit. I own a OBDII-to-USB unit. I own a small VGA monitor for Pi use. A thing that would be useful is a thing that does some networking over the cellphone network, but if it just dumps to my home network when I get home, that'd be good enough.
Here's a niggly bit or me: I start the vehicle and the Pi gets power. I stop the vehicle and the power cuts, leading the computer shutting down suddenly. This is not a happy thing with computers. In fact, I think I can say they hate that, and eventually, the SD card will say enough with this and not boot.
So, the proper solution is to have a power circuit with a battery, that allows it to boot when the car starts and sends the shutdown signal when it stops, but providing enough juice in the battery for the Pi to shut down nicely.
Google told me how to trigger the shutdown when wanted. Just need to figure out how to know what's going on with power.