Powering Thin Client PC from car's battery
It is very unlikely to damage the car, especially if it is properly fused (I'd fuse both leads), and should work OK as the nominal input is 12V. However, the car is a somewhat hostile environment for electronic equipment, because of the likelihood of high-voltage spikes on the 12V supply. I would use a suitable filter on the supply input to the unit, combined with transient voltage protection, in addition to the fuses. The units you mention would incorporate such protection, and it might be easier to simply buy one of them.