As for picking things up.
If you are going a basic
head unit
2 front door speakers
2 rear speakers
2 subs
2 amps (1 for highs, and one for subs with built in x-overs)
Or you can add in a few bass blockers on the door speakers, or x-overs, I mean from that point there are a lot of things you can start to add.
First find out what you want to go with before buying the Head unit. If you are going with the above setup. I would make sure the head unit has enough "Pre amped" (which is pretty standard now) outputs. You will need to have 2 sets plus a sub output of RCA connections.
What this does is it allows you to run a separate RCA wire for each front and rear, right and left channel. So if the music or movie is supposed to travel in a circle from front left to rear left, rear right, to front right it will.
If you get just one output on the head Unit then the front and rear speakers will be run off one channel and will all be mono (front and rear) with each other. You will still have right and left sounds just not a distinguished front and rear separation.
Most Head Units come with a separate Subwoofer output.
A personal secret I have always had good luck with is buying the smaller gauge speaker wire and running it double to each positive a negative of each speaker. So instead of dropping serious cash on a real thick wire go with say a 16 gauge for the door and rear speakers and run both the positive and negative to the positive and then do it again for the negative side of things. Gives more than enough current and also is easier on the wallet. Home depot has decent prices on speaker wire in per foot prices.
If you do the above for the subs run with a 14 gauge or 16 gauges at minimum.
I personally go with a decent RCA wire and run it on the opposite end as the power and remote (remote turns the amps on and off) wire. I never have and never probably will drop the extra cash on "Monster Cable" RCA’s
A friend of mine who ran all Orion and JL ran the cheapest RCA wire he could find and never had interference from power sources. His car bumped very loud and sounded good. I had nothing to say.
Then if you are listening to my advice on the pioneers. Sometimes Walmart etc will have a decent 2 or 3 way Pioneer in stock that will do its job when amped.
If you are looking for Subs I am a little leery of buying used as you don’t know what has happened or what work ahs been done. You can find some cheap Jensen’s at most places Meijer, Walmarts etc. Don’t go get road gear, Sony Etc. Stick with a decent brand or if you are going with lower quality go with the Jensen’s. Now this is only on the sub end of things I have not liked the sound of the Jensen 6x9's or 6 1/2" speakers.
For amps... you can pick up Profile amps for a decent price again not the "best" but prices go up and up for what people think the best is. Also again Jensen used to make a decent amp. It was maybe 100-400 watts and ran in the $50-$150 price range. The newer ones I have not had experience with. The older Blue (machine logo) and the older Blue anodized ones were decent for the money.
Obviously others like, JL, MTX, possibly Kenwood and Pioneer make decent amps but you get into just a bit more for pricing. And again you can always find a more expensive amp. If this is the first time don’t go to huge/pricey.
Keep in mind the THD (Total Harmonic Distortion) of these things and also look for what voltage they are getting the stated amps at. Some companies will give an amp rating at 15+ volts and that can be misleading as the normal alternator doesn’t run that high. So therefore on a lower voltage system the amps will be lower. Some on the other hand are at 11 or 12 volts and thus you might get more in your car.
Keep in mind that I believe anything below 0.09 is not audible by a human so if you look at an amp and it has a THD of 0.07 that’s going to be fine. You get into the 0.09 - 0.1+ range you will start to hear the distortion and may not like the sound of it.
Don’t listen to all these people that run the capacitors on their systems. That’s just a band-aid. What it does is store power for the larger bass hits. Well what if the bass is constant or hits so frequently that the power cant be "stored".... Well then it doesn’t work. To alleviate this problem get a battery isolator. It allows the alternator to charge two batteries.
Link to one kind here:
http://www.jcwhitney.com/autoparts/.../storeId-10101/p-4520/c-10101/catalogId-10101
then install another battery and if needed a voltage gauge. I ran this in my supra and never had the voltage drop below 12. I ran with 3 JL Audio (Dual Voice Coil) W6's, each getting 1000 watts, 2 Pioneer 4 way 6x9s each getting 250 watts, and some 3" Blau highs up front getting 100 watts each. I ran a Profile amp for the front speakers and two SOAT amps for the 3 subs and two 6x9's.
I also had a crossover, Audio Control Epicenter and the voltage gauge. Make sure you get a sealed battery for inside the car.
Anyway as far as head units I personally love Panasonic. It is the #1 electronic company in Japan and for a reason. Their stuff lasts period.