For cheap power I would suggest a desktop power supply (200 -300 watts) and an inverter. Inverters cost about $20 for a 225 watt and $30 for a 400 watt. You will also need to setup a startup and shut down controler to allow the computer to start when the car is started and to allow it to safly shut down when the car is off so it does not drain the battery. (Schematics/Pin Outs/Diagrams)
I would suggest running the cd rom and lcd off of the regulated 12v and 5v lines of the power supply, that way they start with the computer and provides regulated current for longer life.
For audio you will need a 4 ch amp and possibly a subwoofer amp. You will need a front and rear output on the motherboards onboard sound or a 5.1 sound card. You can in a pinch use the single speaker out on a motherboard if you split the jack into two and use two headphone jack to rca converters which would plug into the 4ch amp. The amps remote turn on would require you to run a +12v line from the computers power supplie to the remote lead on the amp. So it will turn on with the computer.
You may also need a fm tunner card if your friend would like to be able to listen to the radio.( since he will no longer have a head unit.)
As for the lcd a 7 or 8 inch touch screen monitor would be ideal and that could be run off his computer 12volt line on his power supply.
You many also want to consider installing a usb hub in the front of the car to allow for usb devices like a keyboard and mouse. If you are planning on running a usb hard drive or cd rom you should use a powered usb hub. This requires you to run a 5 volt line from the power supply to power that and allow it to support usb powered devices.
Hope this helps.