To get the best sound the most important thing to get right is the installation itself...
Door cards are NOT the best place for bass or mid speakers as the card is too flexible and the volume in the door is (usually) too large.
Stereo image (which is where a large part of the "quality" comes from) is created by seperation and positioning...The tweeters (high frequency) and, preferebly, the mid drivers (vocal frequencies) should be positioned as far in front of you as possible...At a pinch, level with you in the doors is acceptable.
Bass is largely non-directional and can be placed in any convenient position...
If you want to fit mids or bass to the doors, then you should construct door pods out of marine ply and have them covered/trimmed to match your door panels. (don't construct out of MDF, if possible, as MDF swells & disintegrates with even small amounts of moisture). The door pods should be sealed at the rear to give the correct volume for loading and tuning the selected drivers.
However, the tweeters can be fixed to the door panels, if required, but the top of the dash or the side of the A-pillars is the ideal place.
Bass bins in the boot or rear shelf (with appropriate enclosed volume).
The actual components are, perhaps, less important.
Head units do NOT have amplifiers capable of producing enough power to adequately power most speakers...The 45w x 4 rating of the (say) Sony units is a peak power (PMPO) rating..usually burst for 0.5sec and at a defined frequency, usually 1Khz and at a whopping 10% distortion...
At best, the real-world figures are around 7w/ch RMS 20hz-20Khz @ 0.1% THD
This is nowhere near enough to derive decent quality and volume levels...
You need an amplifier producing roughly twice the rated RMS power of the speakers chosen...
So assume you choose a component system with a 65w RMS rating...Your amplifier should be capable of producing around 85-130w/ch RMS into 4ohms (the impedence of your speakers) with no more than 1% THD accross the full frequency range.
I have a similar system fitted to both my cars...
The M3 has an Alpine CD/Tuner with a JBL 4 channel amplifier, Vibe front component speakers, Blaupunkt co-ax rears and a Vibe sub in the boot...
The 328 has another slightly older Alpine CD/Tuner, Caliber 4 ch amp, Alpine front speakers, standard Nokia rears and a Magnat sub.
The front speakers are driven from 2 channels of the amp, the sub is driven by the other two bridged and the rears are driven from the head unit (with the balance turned 90% front unless there is someone sitting in the rear).
