On your car, one other modification MB wants done is the addition of a magnetic pad that lays in the bottom of the transmission pan. This entered production I believe around the beginning of 2001, but MB encourages the addition to the earlier 722.6 transmissions if the opportunity arises.
There is one other modification for the addition of caps that cover the electronic shift valves, this requires the removal of the valve body, but this is quite easily accomplished once the pan is removed. Your 99 shouldn't need this, seems like this was started during the 98 model year run or so.
One thing you should take note of is what was mentioned previously: The filling procedure for this transmission is nothing that should be taken lightly. This is a very efficient transmission due especially to the case design which scavanges oil back into the pan quite quickly. Proper fluid level is of paramount importance. To do this the special tool dipstick is used and the trans oil temp monitored via the actual values screen using SDS.
The oil level is checked at 80 celcius. A trained technician will be able to montor the level and keep the level at the appropriate mark and have the fluid level correct as the 80 degree mark is reached. One thing to add is that I have information from a class I took where they state that the latest information from Germany is that the transmission seems to shift best at the "MAX" line on the dipstick. Not overfilled, just maxed out. At the middle of the range or lower, it is possible for the fluid to develop air bubbles, which affects shift quality.
In regards to the fluid, the MB fluid 001-989-22-03-10 is the only fluid you will find available in the US. The only other listed recommended fluid is in effect the same stuff, it is Shell ATF 3403-M 115, made by Deutsche Shell AG Hamburg. You will notice on the label on the back of the bottle is the initials "DSH", which is of course Deutsche Shell Hamburg.