I may have a solution that will give you modern sound with the stock head unit in place. On my 300SD I installed a Kappa K5 Bluetooth Amp. The amp is in the trunk with the sub (Boston Acoustics) and I had 4 Pioneer mids with built in tweeters on all 4 corners. There is a wired controller if you wanted to have handsfree calling, but I opted to keep it in the trunk. If I didn't have to remove the glovebox so often to fix things I would've installed it there. When I had the system installed, the installer wired the power source into the Becker headunit so when I want to turn it off without shutting off the car, I can turn the volume knob all the way to the left to click it off. Pretty nifty eh!
When I get in my car my iPhone pairs and I have great modern sound wirelessly whilst retaining the stock look. Plus, you don't have to worry about some POS breaking into your car to steal your radio. I'm assuming that still happens.
People can't believe their ears when they get in my car and I pull out my phone and start pumping drum & bass!