voltage regulation for car computer
I don't know well how one can connect a pc hardware in the car 12V supply but,
instead of connecting it wich has multiple voltage input and a high power consumption, you can use the new rapsberry pi (pi 3) witch has integrated wireless, high processing capacity, lots of gadgets available and is powered only by a 5V rail;

For the power regulator, you can use a cheap DC-DC step-down buck converter like this:
http://pt.aliexpress.com/store/product/DC-DC-Buck-Converter-5-25V-to-3-24V-Step-down-charger-Module-Adjustable-Constant-Current/715925_854284130.html

This devices usually have closed loop system that assure that the calibrated voltage on the output do not change, no matter the input (while its level is greater than the output). They have great power efficiency and robustness. If you want to use a common router instead of the built in wireless, you may use one of this DC-DC to power it. By my experience, it works fine;

However, RPI do not have sata connections and therefore use it with a SSD its an ugly work.

An alternative is the orangepi (http://www.orangepi.org/).

Keyboard, mouse and audio signal have low consumptions, however look carefully to your SSD, display and SOC consumption before buying the DC-DC converter;