[aseem]

Your prices are way way costly dude. In Delhi the Sony 504 comes at 3.5 k the last I checked 6 months back, in Bangy, I had got a qoute for 3.8k in one days window shopping. The pioneer 6850 comes at 7k. Also, no point going for Pioneer DVC 306 sub, as its stable at 8 ohms. You should rather go in for Pioneer 306 SVC which runs of 300 watt RMS at 4 ohms. So since you are on a budget, you would be able to drive the sub better with an underpowered amp.
The Pioneer sub retails at 2.75 k, a good MDF enclosure will cost around 1k. I suggest going for components in the front, listen to all brands, JBL, Soundstream, BA, etc. I personally have JBL in my car, great clarity, but a little more treble to my likes, BA and other brands suite my taste a lil better. But I wont hesitate to recommend JBL, as Sam would always be here to provide you support if you do choose them. In the rear, I guess JBL GTO 936 should be good enough.
The amp you have choosen, Sony 504, gives no punch. There are better amps available out there, Sony SD series and Pioneer 6100 are good amps. You could use the two channels to power the components, and other two channels to power the sub. Components for Santro in front would require your freind to do some customization, as Santro has only 4 inches in front. However I have seen many freinds fit in 5 inch components after mods. Good luck!