Aftermarket sound system upgrades - recommendations

Apologies if this has been asked a million times -  I’m looking to upgrade from my old mkii vrs to a second hand mkii vrs, but the sound quality of the standard spec seems a fairly big drop off to my ears. Most likely I’m not going to find one with a canton upgrade (not sure how good this is anyway?) so I need to figure out how easy it is/how expensive it is to get good audio from the mkiii

I’ll preface with I have no idea where to even start with this stuff on a car, so I’m after basic recommendations of what’s good, where to buy from & who could fit it - I’m assuming I need a sub, and amp upgrade & maybe even new speaker heads?

thanks in advance