My indy mech says 30K is a safe number. I hadn't seen the 39K number before, but it certainly sounds resonable. Just be sure to drop the pan and change the filter when you do it (don't just allow them to hook up the flush machine and change fluid only or just drop the pan fluid only).
At my "local" dealership (100 miles away), they charge $22 a liter for fluid. I get Mehle (German) with the proper spec for about $8.50/liter at a local parts house. Another thread on this forum says a Valvoline product works just fine for less than $4/qt.
I don't believe your 2001 CLK 320 has a drain plug for the torque converter, so you will need 13-14 liters of fluid to flush all available fluid. Take a look at the procedure on Post #144 in the "How To" thread. If you don't want to get oily, print the procedure and take it to your local indy shop or good oil change operation. You will also need to bring them a transmission fluid "dipstick" in order to make sure the fluid ends up at the right level. These are considered a "special MB tool" and the dealership gets about $45 for them. I see them on eBay for about half of that and a number of internet parts houses carry them. In any case, good thing to have hanging on the garage wall. Also, make sure you get a good quality filter/gasket kit (not a part on which to skimp).
There are no trajectory calculations or hydrazine involved (hence, this is not rocket science).
On the belt, just make sure (translation: "prepare for") having to replace the belt tensioner while you're at it. Also, take a good long look at the water pump while you're in there. I don't know about frequency or the procedure, but these guys will be able to help you better. Good luck.
Last edited by gorgerider; 07-20-2011 at 12:08 PM.