Dude! I bought an HP- E-pc for the same reason - until i realised it was a slow piece of junk when i got it running!
If you have the cash to spend on the asus - looks like a good deal..
All you need to do is get on Ebay and get yourself a universal laptop car adapter - preferably a decent targus or something - although the $15 Australian DOllar item i brought ran the EPC for days with no dramas.
19V is generally a laptop power brick voltage - which is why u would use the laptop adapter.
If your keen on a project, there are plans all over the net to build your own, or buy, a startup / shutdown controller.
This is probably an easier way out than mini-itx / M2 - youve already got a case sorted out, power sorted out on the cheap$$ amd a full warranty on the actual PC itself - not to mentioned its engineered for all components to work well together.
Hope this helps. Wish id seen something like it earlier!
Fighting for peace is like screwing for virginity...
Debating myself if to get one of these machines or not, if anyone knows the answer to the OP's question or even better still have an Eee Box themselves and can take some internal photo's it would be much appreciated.
I am running an EeeBox in my car at teh moment. Works perfectly to be honest. One thing you need to do is update teh bios to allow the auto power on feature. Once this is enabled your laughing as the unit is very small and dosent give out much heat at all.
Basically you enable ACPI on the motherboard which is included with the latest bios update. What this allows is for you to set the machine to auto power on after a loss in power to the motherboard. Thus the scenario that when you turn the ignition off on the car the motherboard looses power from the invertor thus turning off (yes i shutdown before turning the engine off) When you start the engine the inverter starts giving power back to the EEEbox thus turning it back on.....Genius i say.
You dont have to faf about with trying to get it to wake on lan and all that mollarky.