I just saw your post. I've been through two solutions for navigation in my SLK320. I tried a SoundStream VIR-4344NRB (single DIN, non-movable 3-inch diagonal screen) with NaviBox-1 upgrade.
The screen was simply too small to conveniently see from the driver's seat, the volume control on the touch surface was too fiddly, and the screen brightness was too dim for sunlight and only adjustable using the remote control.
I replaced it with a Farenheit TID-897NRB (yes, oddly spelled without the 'h') that has a 7-inch diagonal hideaway screen. This is much better; although I don't like the blockage of the center vents when using the screen, it's nice that most of the time it can stay docked. I can play the radio, and the SD card with full control using the front panel buttons without deploying the screen.
The NaviBox-1 is quick and usable, but in both cases the voice prompts don't work right. I can easily live without them, so it's not a big deal. There is lots of function in the TID. The bluetooth connectivity works fine for stereo audio link to my Android phone. I haven't tried playing a DVD or CD in it but I imagine that it's good. I found the phone function to be workable but less than elegant. Buying a backup camera for an SLK seems like overkill, so I haven't considered using that capability.
I hope this helps somebody with their product selection.