Welcome to the Car Audio Section of Pakwheels.
I would recommend that you buy a good 4 channel amplifier, in your budget a Kenwood or Pioneer would do.
Then buy a pair of entry level component speakers.
Hook that up, afterwards add in a subwoofer if budget permits.
Using a subwoofer with stock speakers would mainly result in not so blissful bass, rather would cause nausea.
Happy Listening!