I've got an install in my New Beetle. Check my sig for a link to the web site of the old install. My new install isn't yet on the site, but the sig takes you to the worklog. It's up and running now.
1. To power your system, there is a cigarette lighter plug in the rear hatch area. Forget an inverter. Get a DC converter for the laptop with a cigarette plug on it and use that.
2. You'll need to come up with a way to turn the laptop on and off as that plug is powered even when the car is off. You can use a shut down controller (available at the mp3car store) but will have to mod the laptop to accept the input. The FAQ Emporium has discussions on laptop vs. desktop and may be helpful here.
3. My old install was in the hatch area and had a Gyration wireless keyboard and mouse back there. Reception was spotty with the keyboard usually working but the mouse not. I moved the receiver to the front and it worked fine. Bluetooth may have better range.
4. Mounting. Consider getting a dock for the laptop and mounting it underneath the rear parcel shelf. That will allow you to slide the laptop in and you can also tap into the dock's power button to signal the laptop to turn off.
5. Audio. I cut the CD changer cable in the rear hatch area and wired it to the PC. It carries the audio right up to the front where the head unit is. I had to replace the VW Monsoon stereo. However, there is a device that is undergoing beta testing that I think will allow you to plug your PC USB into the CD changer cable and it will respond to command sent from the HU by pretending to be the CD changer. It is here..
6. The VAG-com circuitry is powered by the OBDII port, I believe. That means if you leave it plugged in, it will draw power even when the car is off and could eventually run your battery down. You can fix this by putting a relay in line to shut off the voltage when the car is off, but you have to be willing to cut one of the lines going to the OBDII port.