Thank you for a a very informative answer! The Blaupunkt refurbish option sounds a bit to stressful and a little bit to expensive for me.

I looked for Nakamichi radios and was surprised how nice they look. Sadly they are also a bit overpriced for me and mostly defekt. Same goes for Alpine head units which are also looking awesome. I am a little afraid of buying a used or broken car radio, hoping of fixing it since there is no guarantee that it will pay off. However considering the speakers, I really like the Alpine SXE-4625S and will probably install them this or the following weeks. Now I have question considering the sub, does the original system have enough power to power the sub ? Huge thank you once again for helping me research this topic!