Hi, since you have taken all that time to write all that and no ones replyed, i will.
First of all power consumption isn't too much of an issue, as long as you buy a propper DC-DC power supply with a controler, your set.
You see, the power to run an mini-ITX board is bugger all. with that said, i actually have at the moment a micro ATX board running in my car off a 160 watt DC-DC power supply. Yep standard PC parts you can buy at your local IT for bugger all, I payed AU$200 for board & cpu, Ram would be extra.
Here is some tips:
1. Your speaker AMP, make sure its 4 channel, gets you ready just in case surround sound gets you in.
2. calculate your power consumption! for example: 160W (for carpc), 1000W (Sub Amp), 500W (SPK AMP) add them together, 1660W <-- That is how much power this set up is consuming. Now divide that by your 12V battery will = 138.34A <-- That is roughly how many Amps your battery is putting out for your setup. So for this setup id say a 150A fuse at the front would work a treat.
3. Use big guage wire! i cant stress that enough. leave yourself room to upgrade, if you put a 0awg wire though, your set for life. NO LOWER THAN 4awg!
4. Your always going to have compatiblity issues using macs, i have no experience with macs myself, so dont just take my word for it, but software side, with a car pc, you want it to be flexible and innovative. adaptable to new technologies, if the mini-mac can offer that, go for it. I cant see it's power consumtion being much different to a pc.
Basically i wouldnt worry too much about power, your amps will drain at least 3 times as much power as the carpc.
btw power consumption is definetly something to lookout for when building these systems but as long as it's a small board with VGA on board it shouldnt be a problem, still keep an eye on it but dont stress.
Someone else may be able to shed light on the pdx's dont know too much except their class D and small.
have any other questions just ask mate