tranny in my 92 300ce only cost me $1700 to fix, and that included getting all fluids in the car changed, and new springs installed.
I figure I spent on the tranny around $1500 or less.
This was for him to rebuild my tranny. a Factory MB Replacement tranny should cost around what you were quuted if not less or more.
Have you had your fluid and filter changed?
My tranny lost 4th gear completely when I had to have it rebuilt. It did show signs of slipping, but that was it engaging 4th gear then slipping out fo 4th. The moment this happened I took the car to my mech right away.
Upon removign the tranny pan he found the 4th gear clutch pack disintegrated int he pan, and 3rd was on its way out.
I believe the trannies have an adjustment to the bowden cable to change shift points etc. I would have this checked first. Then go for having the tranny pan dropped and fluid changed if there are no signs of clutch pack failure in the pan.
I'm not a tech, but I would search on bowden cable and or transmission slip on this site.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.