Before you change pads, check the rotor thickness. The part the pads runs on starts out higher than the center, and is machined straight out to the edge. When they are worn out, there will be a noticable lip at the outer edge, where the pads don't over the edge, and the center section will be flush the the machined portion. If the rotors are this thin, replace them, since the pads will contact the anti-rattle spring when they wear down if you don't . When this happens, the pistons are forced sidways in the bores, and cock, then stick and gouge the chrome plating and/or drag badly and roast all the rubber bits.
It actually sounds like your brakes are working properly -- if you are actually dragging the tires, slow down some and brake earlier -- it takes some effort to lock the wheels on a Benz, and if you are, you are DRIVING TOO FAST for conditions, and will surely end up like my friend's son -- we always thought he was going to stop just a second late sometime, and he did -- totaled his 300 Turbo hitting a stopped car in front of him! Terrible waste of a car that looked and ran nearly mint.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!