I am by no means a super expert on cars, I have restored a couple, 66 mustang, a couple wranglers, in teh process of redoing a superbee, and was a car audio nerd in HS. Ill just give a quick little rundown, and hopefully it will spur a discussion
audio:
this is pretty general, and the first thing you need to do is sit down and plan your budget. your budget is the determining factor on how you want to approacha new audio system. Never buy an "install kit", find a good electronics shop, and you will be able to find all your inline breakers, 4/8 gauge power wire, rca cables etc, for about 10% of what they charge for a kit at best buy
generally:
100 to 500: this will get you a new head unit, and maybe some replacement factory speakers, a cheap amp to power them
500 to 1000: this can get you a nice sub/amp, amp and new speakers, and a head unit
1000+ all the above, but w/ good components, and nicer amps etc
component speakers are basically 2 or 3 seperate speakers, that have a crossover to seperate the frequencies to the correct part (tweeter= high range, mid = voice, sub = bass), good compontents and a good amp will in some cases get you as much bass as a crappy sub. The most important part about getting good bass from any speaker/sub is the box/enclosure, bass moves air, getting the right sized box will allow you to move the most amount of air, and get the loudest bass. It doesnt matter too much where you position your speakers that are providing the bass, it is very important to get your tweeters about eye level, near your head, this will get you the best sound.
basically, a simple setup, is a headunit, with 2 sets of RCA cables from the head unit to your trunk, and a 4 guage power wire with an inline breaker, also to teh trunk. Split the power wire using a splitter into 2 8 gauge wires so you can power the sub, you also need to run a "remote" wire along with teh power wire so teh amp knows when to shutoff (see your headunit instructions for details), also your RCA cables ran from teh headunit to teh trunk to get the sound to them. Ground your amps negative directly to teh trunk, adn always run your rca on the opposite side of the car as teh power to prevent distortion. than you just run speaker wire from the amps to each of the speakers.
good brands for whatever you need: alpine, jbl, rockford fosgate.
http://www.mp3car.com/vbulletin/newbie/
great site on car computers etc
car mods and restoration:
this is really too big of a subject, so if you guys have questions just post them, and Im sure I or someone will be able to help