Aamco is junk and will probably charge you $3000.
Sun Valley Mercedes Transmissions
OR
https://www.silverstartransmission.com/transmissions
I have heard ONLY POSITIVE stuff from those places. I do my own trans work, so I have no personal experience.
FWIW, you could have a loose pan. Loose drain plug. Leaking cooler hose. Lots of easy spots you may be leaking. With 138k, the trans is fine inside. Last one I tore down had 270k and you could still read the MB ink on the B1 and B2 bands as well as the K1 and K2 drums. Reverse clutches were nearly dead, as to be expected. Otherwise, trans was good to go. Meaning, I would have it pulled, replace reverse, resealed, and install if I was you.
One of our members here runs a top flight shop in on that coast from memory. He is an early AMG junkie, I will find his info and post it. Crap, he is in GA. Sorry. Maybe he knows someone up your way.
http://www.peachparts.com/shopforum/members/jhodg5ck.html