ARM or AVR for car system
Given the potential multitude of I/O and processes this will need to manage, you may find x86 easier to work with for smallish volumes. There are dozens of systems out there specifically designed for thesetypes of applications. Most of them feature multiple PCI express Mini card slots for 3G, Wi-Fi, and GPS modules. They are also designed to withstand wider operating temperatures that are encountered in an automotive environment. And, they have built-in power supplies that can accept wide range inputs, and ignition sensing intelligence. They aren't cheap, but you will likely save enough in the development costs to offset the hardware cost.

http://www.logicsupply.com/categories/automotive_systems

http://www.nexcom.com/Products/mobile-computing-solutions

http://www.advantech.com/products/Industrial-Mobile-Computers/sub_Industrial_Mobile_Computers.aspx

http://www.mp3car.com/