Alec, in component sets, they include the mids, tweets, x-over, and the trim rings to mount the tweet.
Grant, about the eq/crossover. The equalizer will let you adjust the bass and treble at a better level than what the head unit can control it. Usually head units have built in eq's, which are usually set at a lot of bass, a lot of treble, or a good mix of both. And some of the better model head units will have a soft bass, hard bass, low treble, high treble features as well as the good mix of both of them. But with the equalizer, there are usually about 10 bands you can adjust all according to frequencies (bass and treble), so then you can adjust them the way you want to and make it sound as good or as bad as you want to. I am planning on (when I get more money) getting the Rockford Fosgate Symmetry EXP2 system, supposidly the best equalizer out there, and it has 28 bands, compared to the 10 on the others (don't get me wrong, 10 is good, but 28 is just a whole lot better, and harder to work with to get it sounding right), but the Symmetry is a seperate unit, sort of like an amp, but it has a cool display attatched to it that you can put in the center console, or you can mount it on the dash. Here's a pic of it:
Pretty sweet little system. I'd also like to get this little baby too:
This thing converts your stereo signal (2 channels) into a 5 channel surround sound, I could do with out this, but it would still be pretty awesome to have and to hear it in action.
Oh yeah, I have the JL Advantage box (basically like the MTX Thunderform, and the Q-form sub box) and I have a Thunder 6000 10" woofer in there, and it sounds 10 times better than my friends 93 exp. with 2 fosgate 10"s in little truck boxes. I would just stick with 1 10" in the Thunderform IMHO. As for amps, I would definately go with an MTX or a Fosgate, I have both, and I LOVE THEM. The MTX is model RT2200X (harder to find for some reason), and it puts out 100x2@4ohm stereo, and I have it wired into 400x1 bridged 4 ohm mono, and it hits really good. And I have a Fosgate 600A4 amp powering my front 5.25 components, and my rear Pioneer 3-way 5x7s. I would definately recommend this setup is sounds really good. I would also go with an Alpine double din head unit, those things are really nice, but I don't really know of any others, since mine is only a single din size, and thats the only ones I really look at. Hope I was able to help you out a bit.