1. You don't need a GUI to run an mp3car, as for GPS i'm not sure of what is being developed in that area, search freshmeat.net for an answer.
2. It's not just your specs, its also how you configure your kernel but I have cold booted my EPIA800 in just under 10 seconds to playing a sound.
3. Why hibernate when you can boot this quick with a little time and patience.
4. Headunit can do mp3's and dvd's I think and there's also Xcar being developed , both by members in the forums, check the threads for their features but both run on linux ... and yes that means they would work in Debian
5. With linux you can make it do anything you want it to, part of the reason why I have switched to the dark side of open source.
Good luck linux can be a bumpy road to begin with but persevere and you will reap the rewards of total customisation.