I did my 98 E320 myself since the Mercedes Dealers around here wouldn't because the milage wasn't at 100K. It wasn't any more difficult than doing a transmission fluid change on a GM vehicle.
I'm not sure about the E300TD but I have a drain plug in the transmission pan and the torque converter. I droped the pan to replace the trans filter & gasket.
The main thing to watch out for is putting in the correct amount of fluid back in and checking the level at the proper temperature (special tools are required). Also use the proper Mercedes synthetic trans fluid (around $11/Liter).
Do a search for 722.6 Transmission for more information.
My belief on DIY projects is that if you are not 100% confident that you can take on a project then don't unless you are willing to risk the $ at steak (the 722.6 Transmissions are not cheap).
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles