IMO the first step would be to do a proper sound deadening before installing the speaker setups. sound deadening the floor and door would be a great start. perhaps components for front and subwoofer with amp? Budget wise in the end is up to you cuz not everyone is willing to spend alot on ICE. Perhaps 2-3k for a start? 1k + for basic sound deadening and another half for basic speaker setup? then u will go on from there...  or maybe if u feel that ur speakers are fine for now u can add a subwoofer and an amp and from there only u decide

minimise the vibrations caused when u have a better sound systems ie speakers in ur car. as u turn up the volume, the door panel will rattle along with the speaker. so adding foam/sound deadening layer will minimise the vibrations and aso reduce some of the exterior noise. Some people said that sound deadening ur car is almost useless and it doesn't bring much difference. Well highly debatable topic. I think you can get better suggestions reading the forums as I myself are not good at this  Not many ppl active here nowadays. Of course you can spend less than that. I feel that there is solution for every budget bracket 

Simple set up can be done... just maintain your stock speakers and tweets... change your player to perhaps kenwood or pioneer.. see your budget lahh if it permits go for double din if not single din would do the trick... add one monoblock amp and also woofer.. should be just fine.. if you got extra cash.. change your front speakers into a component set