This has been covered in depth loads on benzworld, search search search.
I have written a load about it too.
FO will only work with later system post 2003 IIRC.
The newest ones are now ERISIN Android 8.0 with 8Gb and potentially 4X 32Gb Media (2 SD Cards and 2 USB).
If you go for a decent Amplifier you can forget the F/O and use a 32Gb SD Card for all your Music, dump that ailing and dated CD Player and Navi Drive.
Without COMAND you will not get a Battery Drain, on Post Facelift Cars you may have to code COMAND not present in CGW,
never done Post Facelift
Most all Steering Wheel Buttons will work with it if it is wired up correctly and Menu's configured right.
Steering Wheel buttons for the rest of the Cars functions remain working.
Same for some of the Dash Cluster info, it will display Radio and Track Info.
It doesn't display Nav Info on the dash, but it does have a nice big Screen compared to the clunky old COMAND !!
A load of people say they don't work because they don't understand that if CAN Bus is wired right you do not have to connect up a lot of the wires !! If you blindly connect everything then you will get conflicts.
The Motherboard / Main Board is generic and used in other shaped Head Units, so it comes with these wires, but you simply don't connect them on CAN Bus Cars like a Benz
Final Icing on the Cake is the FM Radio and Navi will work with the good MB Amplified Antennas, the TV and DAB probably will too, I went in depth with that too a while back
Sadly I've not yet tried the TV and DAB as I am too busy right now to finish my Car off
What with my busy Business and the current "Family Crisis" that is now destroying yet another year of Mels' and my own life, after the previous 3 years of In Laws I thought I'd at least get a break
How dumb was I
Anyway, yeah, great units, get one and get stuck in, take pix, post up an Install diary, and if you get really stuck ask and I'll help if I can
Mine is also having 2 Headrests with DVD / Monitors as well as all the Cameras and TPMS, Internet etc when I finally get to it, it is all sat waiting patiently in a box in my Workshop