Been there for the same reason
Roof bars can't be retro fitted, the pillar strengthening has to be part of the build process. Fortunately, the SLK is our weekend car and my wife's car has a SAAB for load carrying. It is disappointing that this stops us using the SLK on weekends in the country when a bike is involved.
I did wonder about adapting a standard saloon/hatchback trunk/boot rack. Since you have no choice you might want to think about that. I wouldn't operate the roof with it attached, also not sure what the additional wind load would do to the trunk/boot alignment!
As for the head unit, I once saw a post from someone who used the cup holder slot on a R171 to stow the screen so it can't be seen when parked. When in use, it slides out and up to vertical.