Fr the proper gunctioning of COMAND with NAVI you must not only replace yur old radio,
but also retrofit new cruise control switch, and integrate the new control unit into your Mercedes CAN system. For that, OEM diagnosis tester is needed with working connection to Daimler headquaters. So i believe iy is necessary for you to find MB dealership, which will be willing to at least program all of these things for you. They will also have detailed instructions ( which i have as well) and exact parts list for this conversion.
So this is definately possible, but you must check that your local MB dealer is willing to do the finishing ´´touches´´ for you.