One data point on MB transmissions is how fast reverse engages.
They engage slower and slower as the clutches wear.

Once you lose reverse, especially at your mileage, it's time to rebuild transmissions.

Without knowing the history of your transmission, 221K miles is a reasonable amount of mileage from an original transmission that lacks a complete history of fluid changes and gentle driving.

Last time I priced them, rebuilds are in the 1200-1400 dollar range.
and you need to get the torque converter as well, to get any kind of warranty.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles