The "Service Rep" is either ignorant or a liar. I'll give him the benefit of the doubt and figure he's grossly misinformed.
The torque converter drainplug was omitted somewhere (it's in a Technical Service Bulletin) in 1999 "...to cut costs...", or similar wording to that effect.Your 1998 should have one.

All materials needed are available online
 

I noticed the place I usually buy parts from online not only has the transmission filter, pan gasket, drain seal but also the dipstick tool, locking pin and MB fluid at $8 per liter so I will probably just order the materials and do it all myself when the weather improves some.

I usually order from these guys:

http://www.maximumautoparts.com/

Free shipping on orders over $50 and I get them the next day in NH.

http://www.peachparts.com/shopforum/702285-post9.html

Gillly copied the bullietin for us. Pay special attention to the 2nd paragraph.

The Automatic Transmission fluid is said to be filled for life. We never specified who's life. (Yes, they actually wrote that in this!-DG)

Here is the easy way to change all the fluid in any auto transmission: Get two clean 5 gallon buckets and 10-12feet of rubber fuel line that is sized to fit over the ends of the transmission cooling lines that run to the radiator. Disconnect the cooling lines at the radiator and connect a length of the abovementioned hose to each line. Place the other ends in the bottom of the clean buckets. Now start the car and then immediately shut it down. One of the hoses should start spraying transmission fluid into its bucket. That is the discharge bucket. It is helpful the have someone else watch the hoses and signal to shutdown once the discharge hose is determined. Now fill the other bucket with the correct transmission fluid capacity plus about 10%. Start the car and let it idle. The old transmission fluid will discharge into one bucket and the fresh fluid will be sucked out of the other. When almost all the new fluid is sucked up shut the car off and reconnect the cooling hoses to the radiator. The fluid is now changed. Check the level and test drive the car. Obviously if you want to change the filter this should be done afterwards but this is the only way I know to get all of the fluid changed in a transmission with no torque converter drain plug. RT

Transmission fluid change
 

In our country 50 % of the Taxis are 0 to 4 year old Diesel Mercs doing typically 70.000 miles a year. The Indy I am using (specialized in MB only - lucky me) told just recently that every Taxi will have their transmission fluid changed at 70.000 mile intervals because Taxi usage is very tough compared with normal civil use. The average used transmission oil of a Taxi looks very dark and there is a lot of grey sludge in the pan because of the graphite remains. Estimated lifetime of the box without changing the fluid and filter is very low - do not try it at home.

Had mine changed just recently as well. $200 for oil change compared with $4.000 for replacing the tranny does not make any comparison. Besides new oil makes me sleep better.

Got my fluid changed today. It was brown to black. When it was draining out of the pan, you could see it was not reddish. This car was from Chicago so I am assuming a lot of start and stop traffic. Also I would drive the car like I stole it so I want some good clean fluid at 70K. Yes, there was no silt at the bottom but just the fluid being black is not good in my book. No burnt smell either. Probably just very dark and not black.