If you change your ATF you should definitely change the drain pan gasket and the filter. I think those filters are only about $15 from the dealers, and the gasket $2-$3. With that job your highest cost, no matter where you buy it, will be the MB-spec ATF - expect to see that in the $15/qt range, and I think your car will need 8.5 quarts.
My dealer charged me about $200 for a "transmission service" which was a partial drain/refill @ 4 qts, filter, pan gasket, lock pin, and 1.5 hr labor.
My dealer charged me about $200 for a "transmission service" which was a partial drain/refill @ 4 qts, filter, pan gasket, lock pin, and 1.5 hr labor.