Awesome job masha Allah.
Gr8 project. Sorry i missed it earlier and couldn't be of any help.
I started working on the same project like 3-4 years back, got me a desktop computer the small branded DELL one its only like 5 inch cube in size. Didnt even need a converter, it came with an adapter type thingy that was to convert 220 AC to 12 DC volts, so all i had to do was plug it in directly to 12v battery power and add a resister to lower down the amps that the car battery gives out..
I faced two issues:
1. Didnt have anything to get display on.. no touch screen was available here.. plus the issue was that all screens available had RCA input rather than vga..
2. Booting time.. everytime i started the engine on / off, i had to give the computer booting and shut down times.. then starting / closing media player, selecting song and playing it.. all things took time..
please share with us how you overcame these issues..