mp3car is good for carpcs (I actually have one, and I go to the mp3car forums almost daily). From having a carpc, I have learned several important things which are often overlooked.
1)sunlight readability. Since you are getting a double DIN, most of them have lcd screens. I have a 400nit lcd with touchscreen and it is almost completely useless under direct sunlight. Be sure to know how your headunit will look in the sun, or you might get disapointed.
2)cold temperatures. Assuming you are getting a LCD, you should realize that LCD screens look very dim in the cold and operating the ccfl backlight tubes in low temperatures will reduce their lifespan. Most of the aftermarket double DIN head units with LCD screens are rated for operation only down to 0C, so you might need to take that into consideration.