I think the answer is in your question:
See "whether flushing the tranny and changing the filter would help ... " and if not "start looking for a used tranny ..."
Be sure to empty the torque converter as well. Check the vacuum connections to the vacuum modulator on the side of the trans.
Hope this helps,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)