What is the best way to upgrade the stereo system on my 1980 450 SL?

I want a CD player, improved/more speakers. I want to keep things as original as possible, but I want quality sound as well. Has anybody installed concealed head units, but keeping the stock one installed for original appearance purposes? Where have you mounted new speakers w/o cutting up the car? ANy help woule be appreciated.

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Chris Meyer
1980 450 SL (99k miles)
2003 Honda Odyssey (Family Hauler)
2006 BMW 330i (daily driver)