car audio ? new sound system?

[professor]

so i have a long list of questions. my friend has a 1997 mercedes benz c230 that he got basically for free, it has stock audio and stereo 2 speakers in the front door panel 2 in the back doors and 2 subs in the back window. The cd player is in the truck but the magazine for it is missing and the car has a stock bose amplifier. The speakers arent bad at all but my friend wants to add some better base to the system. My question is if he wants to do that should he go ahead and replace the stereo all together just because and if he adds base in the back does he need to replace every speaker in the car? Like his idea was to pull the 2 standard subs out of the back window cut a bigger opening and sliding the new subs in making the truck basically the subwoofer enclosure. I know he would need an amplifier for the subwoofers but if you add subwoofers do you need to replace every other speaker in the car ? and im still kind of confused by it all like i can wire a surround sound system in my house no problem but what do you connect from the amplifier to the speakers like i know the amplifier hooks up to the battery which is easier in his car cus its in the trunk but from there does it go back to the stereo and then the speakers? thus adding to the question of replacing the stereo would it be necessary? should my friend just say f it and keep the standard system if its gonna be to big of a pain in the ass for us or should we jsut have someone do it and if we went tht route how much would it cost to have it put in ooo n one final question if he got 4 new speakers and 2 subs how big of an amplifer would he need