Well here's the point, find the circuits you need first 
You will need something like LM7905 and two capacitors to generate regulated 5v from your 12v (all DC) source.
As for controlling the 12v for the computer, you'll probably want to look into a solid state relay (unless someone has a better idea?).
In both cases all the circuits should be widely available on the net. I'm pretty sure you can find both on http://www.epanorama.net/, but google is your friend, as always. As for a pin count, you have more than enough for the serial LCD and the relay(s). You'll probably want to had a few buttons or a keypad too for control (play/pause, next, prev, etc) and maybe an analog input or two for the lcd and volume. If you ever run out of I/Os, have a look at the Shift register tutorial and keep in mind that it is possible to do the same with inputs too. Lastely, if you haven't already I'd suggest you have a look at the Wiring electronic ref, for some quick examples.
And obviously, check the forum and playground a lot of common answers are posted there.
Have fun ^-^