Sound System Questions, 5 quick questions :) Options

[JonCars17]

I have a few sound system questions.

I'm thinking of adding a sound system to my car by the end of May which gives me enough time to save up.

Q1:
What is the point in putting a Sub (8") in both side doors?

Q2:
How much should I roughly look at spending for the system (components in the front, 2 speakers in the back, headunit and a sub 10" with an amp)? I'd like to get one for qaulity and not the extreme bass.

Q3:
What should I look at when choosing a system? the Sensivity aka the dB?

Q4:
How do I calculate how much amp I need

Q5:
What can I do to stop the car from rattling once it is installed?


I'm a n00b when dealing with cars -_-.

[JonCars17]

kindof late but thanks anyway

this is the system I'm getting:

CD Player:
Alpine CDA-9856
Price: $200
RMS Watts: 18 Watts, Peak Power 50 x 4 channels

Component Speakers:
Alpine Type-X SPX-177R
Price: $450
RMS Watts: 75, Peak Power: 300

Speakers:
Alpine Type-R SPR-17LP
Price: $150
RMS Watts: 50, Peak Power: 250

Sub:
Alpine SWR-1242D
Price: $200
RMS Watts: 500, Peak Power: 1500

1st Amp:
Alpine MRV-F545
Price: $650
100 watts RMS x 4 at 4 ohms (150 watts RMS x 4 at 2 ohms), 300 watts RMS x 2 bridged output at 4 ohms (4-ohm stable in bridged mode)

2nd Amp:
Alpine MRD-M1005
Price: $550
RMS Watts: 700 x 1 at 4 ohms/1000 x 1 at 2 ohms

I've already bought the 2nd Amp (mono amp), Head Unit and the 12" SubWoofer.