A DC-DC PSU replaces any other PSU your computer already has.
I would recomend reading this FAQ:
and this one:
Why would you want to use an inverter to convert 12v DC to 120v AC to only have it converted back to 12v DC?
Regarding the screens, there's an entire sub-forum dedicated to monitors on here including some stickies for each brand of monitor. I'd really recomend reading up in there about the different monitors.
As for purchasing your monitor, why not: http://store.mp3car.com?