snapon is a pro

the op didn't go to dealer s/he said 前后去了修车厂3次.  i doubt if the shop really knew mercedes.

for transmission, there are some spots prone to aging:

1) at rear, there is a kickdown signal plug, seals could be toasted

2) at passenger side, k1 and k2 piston seals; kickdown cable seal; dipstick tube seating and pivot pin.

3) at driver side, control module seal and pump seal

4) pan gasket

all of these are subject to aging but diyable. i replaced all of these seals over years.  the only thing tricky was pivot pin - there was no way to take it out even from inside as it's hammered into the tranny body; thus i *****-welded it from outside. 

mercedes tranny really lasts long, most will last 20+ years with proper care (not sure the most recent ones but i think the same).  mine is already 25 years with 174kmi, it runs smooth and still my daily driver.  in the event it breaks down one day, i will have a shop rebuild it for $1100 with 2 year warranty.  it'll last for another 20 years.