karthick why don't you use the existing headunit get a 4 channel amplifier with hi level inputs like the HELIX or FOCAL,get a good pair of components for the front,the similar series of coaxials for the rear,and a sub.This should sound good enough.
If you feel the need for an upgrade later,look into the options for the headunit.
keep some budget for damping like dynamat,focal,etc.