If I remember correctly, you already have vented disks on a 420.
Take a look at the rotors -- is there a distinct lip at the outer edge where the pads don't complete cover? If so, they are too thin and needed to be replaced when you did the pads.
I suppose you could re-use the pads, but unless they show very little wear, the $35 or so spent on new ones would be a good investment. New rotor will last longer with new pads, the wear is more even at the beginning.
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!