Use the original calipers, the others won't fit. You have PLENTY of brakes, these cars will stop as well as any will with factory brakes. If you've never driven on the Autobahn, you may not understand -- NO German car will have brakes the end user can improve, if they did even more people would get killed! The factory brakes will haul that CD down from 110 mph is less distance than you might believe,
Rebuild the original calipers and make SURE you have good rotors -- if they are thin, the pads can jam and stick the pistons -- resulting in very poor brake operation. Mismatching calipers and rotors, or calipers and backing plates will cause you endless trouble.
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!