I had a similar problem with my S420 at 87,000 miles. I ended up going with a factory rebuilt transmission that carried a two year warranty...and I have had to use the warranty!!! I do not have the invoice in front of me, but I recall the transmission cost @ $3,400 plus a torque converter @ $800.00. Labor was another $2,000. It's scary...but it's a W140. You should be able to do much much better than that.
On my car, MB recommended timeframe for changing the fluid is every 100K. Big mistake...every mechanic I talked to about this says to change it @ 50k. Live and learn.