ice_wise: Thank you for the suggestion. I hope I can go with JBL GXA644SI and for the components what do you think of this one - JBL GTO 509C ? They have pretty good rating also when I checked in multiple sites. I'm worried if these can be directly connected to the stock HU without much wire cutting and extra converters.
Let me clarify on the install part -
You have the stock HU. Since JBL GXA644SI has hi-level inputs, it will be directly connected to your HU through speaker level input without the need of a converter. If you go for some other amplifier without hi-level input, then only you will need the converter. About wire cutting, no wires will be cut if you use couplers (a special type of connecter particular to each car which fits the stock wiring harness without wire cutting). Adding speakers directly on HU is a plug and play job. Powering them through amplifier will require extra wires but will not involve EXTRA cutting or modifications at all.
Simplifying all the above, amplifier wiring kit will connect your amplifier with the car battery (for power) and the HU (for music signals). Speaker wires will run from the amplifier through the door sills to the front doors for the front speakers, and same for the rear speakers. Speakers will be added on spacer rings through couplers, which would not involve any stock wire alteration.