Polo II and audio upgrade

Hello guys, I am actually more on the road in the Opel Vectra-B forum, but have a question about the polo (Polo II, first registration 12/1992, Steilheck) of my friends. After the pram no longer needs to be transported and thus the storage in the polo can find its place again, my wife wishes to be able to listen to some reasonable music in the car again (boxing in the front are quite in the bucket). Unfortunately I am subject to a certain quality requirement, since my wife singer i I would like to replace the original loudspeakers of VW with an inexpensive solution with quality. But it doesn’t have to be from VW, but should fit without mechanical change of the brackets. I’m not sure at the back: I’m better to put something in the rack (2 loudspeakers) or a subwoofer in the box in the trunk. Maybe then another pa would have to be put in the trunk. ar tweeter in the back? Since I have not put so much value on the audio technology in the vehicle for a few years, I am not up to date and need support. Would be nice if I would get some suggestions/tips from you how to solve it best/favorably.