About the price point where plethora of choices can confuse.
It depends on the music you like and the ammont of cabin/trunk space you are willing to sacrifice.
Someone who is looking for BOOM
for example can...
spend 6-7K on the HU with one or more preamp outs (as above)
spend 5-6K on 4 speakers for the front and rear (as in the 10K system)
get a economical 2 ch amp and sub (Sony 552 and Pio 12" sub) and a pair of decent RCA cable (Monster Interlink Jr) for about 8K.
Someone looking for resolution
Spending 9K on a HU with more features (such as USB and Ipod compatibility) and better sound quality and 10K on front components and rear coax speakers. It also starts becoming essential to listen to multiple brands and combinatons before buying ICE at this price point and above. Brands recomendations are still the same as those for 10K and 15K with the addition of the Sony amps and Pioneer subwoofers and Clarion HUs.
At this point one must consider a good amplifier kit along with better RCA wires. Such a set of wires can run about Rs. 3000. Monster, Twister, Audison, etc.. all offer such a kit. As a rule most members have not been happy with the BOSS kits in the Indian market so treat these with caution. 4) 25K
Now you can afford a bit of both BOOM and Resolution or more BOOM or Resolution!
For More BOOM just get the Sony 554 amp instead of the 552 and one more Pio 12" (see 20K BOOM system above). There is also a 4 ch. JVC amp I am told is now available but I have yet to hear it leave alone use to drive a subwoofer. Still it might pay to give both (Sony and JVC) a listen. Other 4 amps one can consider are the Pioneer 6300, JBL GT5-604, Blau Gt4 Special, Kenwood 8401 and Gronud Zero TA4000. These will cost about 7K each but are a definite step up from the Sony 554.
For more Resolution consider a better HU (all the brands mentioned above) but Pioneer, Clarion, Alpine and a few Kenwoods might be your best bets. For speakers with more Resolution consider Infinity, Illlusion, Hertz are good choices.
For a bit more of each (BOOM and Resolution). Consider a Ground Zero, Hertz, JBL or Alpine sub instead of the Pioneer in the 20K BOOM system and consider changing the front speakers to components. Your budget will look something like this -
8K for HU, 7K for 4 coax speakers, 10K for JBL/Alpine/Hertz sub and amp OR
8K for HU, 10K for front compoents and rear coax speakers and 8K for the Pio/Sony sub-amp combo used int he 20K BOOM system. I told you can get confused!