1. Okay, new to the xBox scene, but not to car electronics. So, I want to replace the PSU with a DC-DC if possible because it uses a PSU similar to any desktop PC. I know it uses a 110V PSU, but I am unsure of what wattage consumption the system uses, I believe 250 should do the trick. I would like to stay away from noisy power inverters to have big draw on a 10-14v system which is why I want a PSU.
2. Also I have to fit a DVD Drive into a 1/2 DIN slot (half a normal car stereo head unit size). A slim-line optical drive (laptp dvd drive) will fit perfectly. Can the X-Box support different DVD drives? Same brand or not necessary?

Answer to number 2-Usually not because xbox dvd drives have different firmware and it dosnt have the same power imput as standard dvd drives. If it is possible you would have to write firmware for it and do some other modifications so that it would actually be able to connect to its power source. I know it is possible with some drives (a certain samsung model and a few others) but I dont think the modifcations would be possible on a slimline drive.

1.) Total, the Xbox uses about 100 to 150 watts. If you wanted to use a PC power supply to power the Xbox, then it would be easiest just to buy one of these. I'm sure you could wire it into a car if needed.

2.) You can put any DVD drive you want in the Xbox, as long as you don't care about reading original games. Only Samsung SD-616Ts and a couple of other drives can be modified to read original Xbox discs, but none of them are of the slim design. Also you need, as far I know, to remove the DVD drive check in your BIOS so the Xbox can boot without a DVD drive (or in this case an unofficial drive). In addition, you need to get a molex Y-splitter like this so the drive can have power.