Powering the car PC setup is very difficult step and has to be analyzed well before anything. If you have not planned how many devices you are going to be using and bought a very low power PSU. And later you have added a few USB devices and some HDD, then one day the PSU may not be able to handle the power demand. Eventually it will die, worse it may harm your Car PC as well. While choosing dimension of the PSU is also one criterion. I will leave freely to be chosen by those who setup.

Power Adapter: will be required for testing the PSU at home desktop where you setup and test your Car PC.
Battery: will be required for using in the car.
Cables: Choose power cables with proper gauge, otherwise it may lead to fire accidents.
Connectors: Mostly least bothered item in the Car PC setup. If this is not there, the possibility of frying your PSU/Car PC is very high. Choose a connector where it always prevents you from connecting power to the wrong polarity. Consider the connectors at your desktop setup place as well.
Regulator: When it comes to regulator. The expectations are not only for 12v rather it is for all the voltage levels offered by ATX.
Startup / Shutdown controller: Already discussed in another article here. Basically should be smart enough to shutdown and startup the computer with configurable delays.

To calculate the total required power for your System, you need to find how many devices you are using and how power hungry they are. Following are just examples, it may goes on based on your requirement
Motherboard: try to choose a mother board which is powerful enough to handle your system. At the same time make sure it won’t take too much power. Always passive cooled processor and chipsets are best for Car PC as they are not only power efficient but also silent in working.Fans: As the above point if you are unable to find one with passive cooling. Then active cooling is required, then you need to calculate which fan is going to be used and how much power is required for themCPU: the amount of power required by a processor.
RAM: It depends on which type of ram and its capacity. And also how many RAM modules are required.
HDD: It depends on which type of HDD and its capacity. And also how many Hard Disks are required.
Other devices : External Hard Disk, Flash drive, Optical drives, GPS receiver, IR (for remote controller or what  ever), USB hubs, SD/MMC memory card readers, LCD monitor, Basically whatever that is connected to the PSU directly or indirectly (like connected to motherboard, connected to USB hub).

Allowed input voltage should be considerably low as possible for minimum. This prevents your Car PC from rebooting engine start, engine cranking.
When the engine is on high speed for long time like while driving highway, the alternator charges the battery and after some point of time the voltage will be around 15V~(even18 volts). So we need to ensure the PSU should be able to handle that voltage. Normally I would prefer it should handle up to 24V (and I hope few PSU are able to handle that)