My initial thoughts included an interface with the car's OBD2 interface but I though it would be a bit hard to get the protocol right.

But this changed a bit as I found that some people were already doing this on arduinos (OBDuino, there's a lot of info on this) and the opengauge project (https://github.com/magister54/opengauge) so maybe there's hope on this chapter :)

I'll check if I can convert this to pure C code and use it on this module.

For the electrical part I was thinking of using a cheap OBD2 bluetooth dongle and removing the bluetooth module so I could interface the UART pins directly from the Stellaris.

My Audi A4 doesn't have a CAN interface but instead K-LINE (ISO 9141-2 protocol) at about 10kbps so the update rate isn't so good. It will have to do...

More on this latter...