There was an issue with the "plug", which I think there is a better name for it. I think some were leaking fluid. Most people agree that the trans should never be "flushed." Some believe that it is better to leave it sealed than to change the fluid. But most seem to believe it is better to change the fluid. I thought the second mechanic sounded pretty honest, and he was going by what was originally published. I would give him the new info and get a second response and quote for the fluid change. BTW, the cabin filter is pretty easy to do on a W202, I would assume not too bad on a W210. The oil change should not be too bad, although the plug might be hard to remove if it has never been loosened before. Mine was, but is a different engine. A lot of people recommend the vacuum system for the oil. I drain mine.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra