I've done quite a bit of research on carputers, but not much on the wiring or anything with remote start or unlocking doors. The temperature range you require would necessitate the "industrial" grade chips, which I'm sure Atmel makes. If you take the Arduino board approach, it is simple enough to purchase a drop-in the replacement chip and put the Arduino bootloader on it, but you still need to worry about the other components on the board. I'd suggest performing some environmental tests in which I would gladly help out with.
The carputer stuff I'm wanting to do in my own car is to have a built-in 7"-10" touchscreen running android that integrates with the OBD-II and stereo systems. The stereo wiring harness costs 150$ for my '05 Accord but I'm probably going to create my own. For the OBD-II communications, you can buy a device from scantool.net for about 80$-150$ but I've been wanting to roll my own either by snatching the ELM327 firmware and programming my own PIC for far less $$$. The ELM chips can be bought separately from http://elmelectronics.com/obdic.html
for 32.50$. The OBD-II interface can modify engine performance as well as report many of the sensors for logging, analysis, etc.
Bluetooth: do you just want to do locking/unlocking and remote start? I'm sure you can find "open source" plans of rolling your own device to integrate with this functionality in the car. Once the method is actualized, it's simple enough to implement in the Arduino. It would be cool to use your phone to implement this functionality and to also provide speakerphone and navigation capabilities. Also, if you've not seen it yet, I'm creating an Arduino+Bluetooth guide on the wiki with the plans of becoming a precursor to an Android+Bluetooth+Arduino "SCADA" project. Incomplete wiki page: http://www.tcmaker.org/wiki/doku.php?id ... th_project
I've also recently acquired the Cobra iRadar 105 which is a Bluetooth radar detector that pairs with the Android (model 100 for iPhone) and allows configuration, reporting, etc. through the "iRadar" app (free on Market). I've been meaning to reverse the protocol between devices to roll my own app that shares in real-time the presence of radar hot spots with other phones; it would be something like trapster but without having to interact with the phone. The part I'm currently hung up on is implementing a speed limit database in the app for better intelligence.