Minimally, you need the navi "drive" which installs in the MCS above your tape player, then when you lower the door (screen) on the MCS (Radio) you see the cassette deck AND a slot to insert the CD with map data. Here is an ebay auction with a drive so you can see what I am talking about eBay Motors: MERCEDES ML NAVIGATION DVD NEW MAP MCS DRIVE CD NEW (item 290217797380 end time Jun-24-08 15:14:11 PDT)
There is some question as to antenna connection for the GPS to work. I have determine on my 2001 ML I need to add a splitter underneath the pass seat where the tele-aid unit is located, to get the antenna split and connected to the cable that runs up behind the MCS.
Does your 2000 have tele-aid? Do you have the small black antenna on the roof near the rear hatch?
Try some searching to see if anyone did the exact swap in a 2000. Seems for 2002 and newer, the antenna situation is resolved (meaning no work to do) just pull the MCS, add the navi drive to it, re-install and you are all set.
I am not sure the dealers are too open to giving advice, but if you have a good service department at your local dealer, you might ask them what is required, specifically the antenna piece.
Let us know what you decide to do.