1) there has been alot of talk of trying to get headunits to hook up to computers through the cd-changer controls, and most people havent been able to do much. i think i heard something about getting it to work with a sony, maybe. do a search on here for "cd changer sony" and see if it comes up with anything.
2)As long as computer parts are protected from dust, water, and as long as the sun doesnt get it to 40+C then its alright to leave them in the car. Make sure you have them secured down, vibrations will only hurt if they knock something loose. Harddrives are the most susceptible to vibrations, although most people here dont have problems. if youre really worried about it, laptop harddrives are designed to be more rugged then desktop drives.
LCD screens will be really sluggish in the cold, but once they warm up then theyre alright.
3)If you have experience with power supplies/circuit design/construction, then there are some here that can be made. search for sproggy, i believe it is taking a regular supply and converting it to be dc-dc. theres a lot of discussion about power supplies in the powersupply forum.