you can get a wiring harness from becker of na and upgrade to a newer factory cassette of the MB optional cd player or an audio 10 unit from ebay.... or if you dig in your pocket deep enough you can bet a becker monza mp3... if you want to stick factory and factory look that is...
I too don't like the flashy stuff, and I found and installed an older alpine cd player (mid 90's model) in my old w123... you might consider that option especially if you wanna retain stock speakers and fader....
here's some examples on ebay
monza
audio10
older alpine unit
alpine 3cd indash changer