Any autobox will start to slip if abused.

Check it engages drive and reverse without too long a delay and no nasty clunks.

Take it for a test drive and see if it flares when changing from 2nd to 3rd and 3rd to 4th etc....

Check fluid condition, is it clean? Does it smell burnt?

I don't believe MB gearboxes are any weaker than any other manufacturers, like all auto gearboxes, they need regular oil changes and a degree of mechanical sympathy in how you treat them as they get older.

Good Luck,

Richard

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1989 W124 300E
1991 R129 500SL