The main problem is that it's hard to decide if it is worth it these days with other commercial solutions on the market for this sort of thing. Whether these devices are up to the job really depends on what you are looking for. Things like storage capacity and media types (TV, DVD, etc) are really a matter of personal preference.

There are a couple of in dash CD head units available now that will play MP3's from a CD. Some places I have found also sell in-dash MP3 head units based on hard disks.
So if I decide to do another one of these things how would it be done? Well, in the intervening time since I did this project there are lots of new and exciting products for this sort of thing. In particular, mini-itx motherboards that have just about everything onboard and measure 17cm square. Also there are 12v to ATX 200watt DC convertors that would make life way more simple. Further, if a laptop hard drive was employed, there would be no need for the 12v regulator I use at the moment, as the only thing left to need 12v would be cooling fans (where precise voltage is less important).

There is *even* a compartment in the car dashboard that I don't use which could take all of this stuff, meaning no ugly MDF box in the boot! :o) So the idea looks good, its really just a matter of weighing up whether or not it is worth it these days (money and time!). I am thinking what features I really want from the finished system and at the same time considering what head unit I would buy in terms of sound quality/cd changer control. There is also other factors such as if I want this to be a player permanently fixed in the car or would it be cool to have it 'dockable' etc.