I say, that whatever makes you enjoy your car more will make you want to preserve it more. Which means you will probably take care of it better. You can always upgrade the head unit and speakers but keep the old crappy components if you just like bad audio for nostalgia's sake.

 

I suggest hiding modern audio equipment controlled by your phone and leaving original head unit in place. It doesn't matter where the head unit is if you are controlling remotely by bluetooth or future tech. http://www.outofsightaudio.com
In the centre console between front seats might be an option if you want an accessible unit too?

 

just replace the stereo with a modern one if you play CDs. Leave it alone if you play Cassettes.

I still play cassettes.