Head Unit Suggestion
Every brand (Alpine, Pioneer, Kenwood) have their own signature sound. Its a matter of personal preference.
Personally i like Pioneer Head units, they do sound warm to me. Kenwood Head Units IMO have a very forward sound.
Speakers
Add components speakers in the front. I dont think you need a rear fill. But if you still want a rear fill then forget the 6x9s if you are adding a sub, 6.5' coaxial wud suffice.
Subwoofer
1. JL W6, 2. Pioneer PRS, 3. Infinity Reference, 4. JBL GTO. (1 and 2 being most preferred)
The recommendations are mentioned keeping in mind whats available in Pakistan
Enclosure
For accurate response go for a sealed enclosure.
For loudness go for a ported enclosure.
Remember for SQ setups the sub is just for the "fill-in" i-e the sub is only there to compliment the front components speakers and to fill-in the missing bass which your comps cant play. (i-e from the cut off ferquency say HPF 80 hz or 120 hz and below)
Aim for a sub which gives accurate response at comps cut-off frequency and below. In simpler words you need a sub which can play 120hz or 80hz frequency notes accurately and below.
NOTE: If a sub cant play accurately at the cut-off frequency of the comps, that sub is by no means a SQ sub.
Amplifiers
Don't buy any amp before you purchase the sub and components. The voice coil configuration of the sub will determine what you need.
IMO if you are going for the rear fill, there is no need to power it with a separate amp since the stage might shift to the rear, which you do not want. You can power the rear speakers with the Head unit.
Components
Range..! Particularly for components ???
Below 10k Pioneer comps are good, under 15k Alpine Type R, Infinity kappa, MB Quart comps are good.
Battery
No need to upgrade the electrics, its not a SPL install. But i wud advise the BIG 2
Hope this helps.