well, it has been covered time and time again, but....
For your laptop, look for a docking station, and mount it solidly in the car. Then, just dock it and drive away. You can run a monitor, keyboard, mouse everything that a desktop can run. Usually have good USB support too. Get a touchscreen lcd...like a Lilliput or a Zenarc and Run a cheap inverter to power up the dock. Unlike running a desktop style board off of a inverter, you have a battery for a filter so you should be ok. The touchscreen will take away *most* of the need for a mouse and keyboard.
So basically, round up a dock, inverter and a monitor, and download frodoplayer, mediacar, media engine, or one of a handful of other players and your set.
The LCD connects to the VGA out ont he laptop and the touch screen connects via USB. I would recommend using an external soundcard as laptop ones are generally pretty rubbish and only 2 channel. As for gettingt he sound out of your speakers you can either connect the soundcard to your head unit if you have aux inputs or instead you can buy an amp and bin the head unit (in which case you will want a PC radio). Another advantage with USB soundcards is that you can mount them very close toy our amp/head unit to keep the audio leads as short as possible.
Best thing to do is look around at completed laptop based projects and see what people have used.
If you are looking for some visual type images, check out my website www.samshomepage.net It has pictures of framing and installation of a "desktop" type pc into my car(s). Version 1 and Versio 2 are my old 1988 Honda Accord (thank God it's gone). Version 3 is my new car. Looking at the pictures might help you understand a little more. Also, on version one and two are parts lists of what actually went into this system. Laptop would be easier with a docking station if you're just starting out. But I prefer the ability to change and upgrade all my components that you can only get with a "desktop" type set up.