If you go ahead with the CarPC, my only suggestion is to take your time reading up on the forums here, and keep a papertrail of the parts you want to use rather than actually buying them. Then once your satisified with the setup you have when your done, order the parts all at once and move onto a testbench to get the system working to your liking before ever even installing it in the truck. After that is pretty much up to you what you are going to do and how your going to mount everything.
Otherwise, if all you really need is mp3, dvd and backup camera, just go with one of those prefabbed retail brands, get the hookup on the install, and get instant gratification a few hours later when the install is finished and you didnt even scrape your knuckles. There is no way with a CarPC you will ever get the satisfaction of the job being "complete" since there will always be another tweak or part that you want to upgrade to later.