The nav system install into the MCS system (Modular Control System, that's the radio on a 2000 and newer ML) is fairly easy.
The screen isn't replaced, the nav system and CD driver are a "drop in" to the MCS unit. I'm trying to remember for sure, but I believe there may be a small amount of work to be done under the passengers seat, like a antenna switch or something. I have the instructions at work. The nav driver is accessable after installation in the top of the radio when the screen is dropped down, like if you're going to load a cassette tape, but at the top of that panel, you can see the dummy plate on the top.
The truck is already equipped with the nav antenna, the truck has it because of the Teleaid system that is standard on an '01 uses it. There are only a few hitches for a do it yourself project. One is that the MCS has to be removed to do the drop-in. You need special tool "keys" to pull the radio, you can either buy them, or maybe a shop in your area will either pull the radio for you or let you use their keys for a moment to do it. Another is that the MCS unit itself has to reprogrammed using the MB shop computer to reflect the internal change to include the nav system.