components now? subwoofer later? 
if you're getting components for the front (and no coaxials for the rear), please know that you WILL need a subwoofer for the bass... the components you get in your budget will not be able to handle any real bass... they arent really meant to.
decent components are for 7K and above (but you can search PW for good used ones)
plus you'll need an amp if you're looking to get good sound out of your speakers. a 4 channel amp if you're looking to add rear speaker or a subwoofer. search PW for used unrepaired ones. there were plenty of them here the last time i looked.
if you're looking to add 4 speakers (coaxials or components or both) plus a sub, you should make sure your system has 3 RCA outputs and then you'd need another 2-channel or mono amp for the sub.
my recommendation... if you're low on cash... build upon your system gradually...
i first bought rear 6x9s... then added a 2 channel amp... then added components in front plus a 4-channel amp... then added a sub.