Gosh, where to start?! One good online resource is http://www.mercupgrades.com/R171+SLK-class-24.html
- loads of good info there....
If you read the history, MB has complicated their lives by calling various generations of COMAND "COMAND" or "COMAND APS". The link above will go into gory details.
NTG2.0 is what you have in your car. NTG2.5 is what the new, facelifted R171's (MY09) come with. Navigation is no longer an "option" on the NTG2.5's,. it's integrated. The "Changer" is now the option, as it's built into the head, so no longer a dealer option.
There are various software revisions to the COMAND NTG2.0 you have in your car, and the - honestly - easiest way is to have your dealer pull the data from it using DAS. Complicating things even more is the fact that your car has a separate navigation processor in the trunk, and obviously - it needs software as well!
MB uses one of two Navigation data vendors for the US market, so not all MB models have interchangable Map Disks (they are based on different navi processors as well) - I don't know off the top of my head if the ML is using the same as the SLK - in general the models supported are printed on the top of the disk.
There was a major firmware upgrade for the Navi processor in MY07 to support better route management, a few other fixes, and also a change in the data format. If you put too "new" a disk in the Navi, you'll get a "Can't read disk" errror - and your dealer should flash it for free.
Most minor Navi updates are actually on the maps disk themselves, and will tell you that they are updating, etc.
So - short answers is that Merc Upgrades has the best info available outside MB. Yes, newer maps can require a firmware update on the Navi processor to work, minor included on the maps disk, major from the dealer...