Unsure about your monitor choice! Is it touchscreen capable? How much does it cost?
I also use a laptop to power my system, and it is a good choice. Why not mount it in the trunk however? As far as DC power goes there is only one way to go in my opinion. Using a DC to DC converter. I went with an I-GO 7500. It plugs into a cigarette lighter, and will convert 12V dc to whatever your laptop needs. There are other similar things available on the market, but you have to get a converter that does dc to dc if you want the sound to be right. It has different tips for the different models of laptop.
Powering on the laptop from the trunk is a little tricky. The best solution in my opinion requries a little bit of soldering skill. You solder connectors to the power button on the laptop, and connect those wires to the 56K modem points. Then string a phone wire to where you want the power button mounted. This adds a phone wire from the laptop to the power switch as well as the VGA,USB, and sound to the amps. As long as you are running wire you might consider running wire for a microphone so you can use navi-voice or if you have a bluetooth phone you could use the mic for phone control. Even if you don't use it right away at least you only have to tear your car apart once right!
By portability I assume that you mean you can take it out of your car easily right? If you really want to take it in and out then the laptop is the best choice. PC users have the upper hand because there are power supplies that are built specifically for them that control the startup, and shutdown of the computer without having to press any buttons. This can be done with a laptop, but requires a lot more knowlege like the mod I talked about earlier. Laptop is cheaper if you already own the laptop.