Michael,
I have a Becker Europa 2 (a stereo radio that is newer than my '66 230SL). I have purcahsed from Konigs in Germany a lead that plugs into the back of the radio which has a 3.5mm jack for an MP3 player. Copying another member, I have brought this lead from the back of the radio, along the transmission tunnel under the carpet, and into the ash tray where it sits when not used. When I want it I just pull out the jack and plug in the player. Alternatively, I understand an Ipod nano will fit in the ashtray.
I am currently planning to put a small amp under the drivers seat to provide better sound with the top down. My speakers are in the footwell an I have no plans to install others. Depending on the amp in your car, you could run an mp3 player direct to the amp, or just run it from the radio you purchase assuming it has the spare 7 pin jack at the back. I would keep the amp.
One more thing, if you are going to change your radio for an original, note that again depending on the amp, you may need to by a line converter (about $15) as the Becker radios do not have pre-amp RCA outputs. Instead, they have speaker outputs. It may be worth checking the specs of your amp to see if it accepts speaker inputs (also known as high level, i.e. amplified, inputs).
JH