Here is an excellent thread on changing the fluid in the 722.6:

http://www.peachparts.com/shopforum/showthread.php?t=259116&highlight=722.6

It's not really much different than changing the fluid in any of the older MB models. You just need a few more specialty items such as the dipstick, specific ATF fluid, and plastic pin to secure the cap.

I know that my 140 has the drain plug on the torque converter. I believe it was eliminated in the late '90s, though, so yours might have one. Seeing as how that first mechanic never recommends changing the fluid or filter he may be wrong on that as well.

I changed my 722.6 fluid myself, but just out of curiosity I checked with the local MB dealer to see what they charge. It would have been about $230 to have them do it. The response from the second mechanic seems expensive. Why replace the electronic plug and "rear end" if there's nothing wrong with it?