Minimum for pads is probably 3mm, or when the brake wear sensor light comes on (front pads only) when you step on the brake.
Front rotors on the W124 are held on by the lug bolts (finally) so they are easy to change -- remove caliper (two bolts in the back), remove the small hex socket locator bolt, and remove rotor. Probably rusted in place, you may need to thump it with a hammer once or twice.
Press pads back into caliper after removing the cap on the master cylinder reservior (they wont' move with the cap on, it's nearly sealed!). Two types here, some are the old twin cylinder ATE/Bendix and some are, I think, floating calipers. I've not had to replace them yet on my W124 chassis cars, so don't know for sure. The ATE type, you knock the two retaining pins out, remove the spring, and pull the pads out by the "ears". Push pistons back into the caliper with a large prybar or a C-clamp -- make SURE you are pushing straight back! They stick if you push crooked. Clean the slot where the pads go, and put some antiseize on the back of the steel plate and some on the sides of the backing plate only, this will prevent squealing. Do one pad at a time, otherwise the other piston will pop out!
If they are floating calipers, I think you hvae to remove one of the bolt pins and flip the frame up. Pads will have a littl spring on top, clean all sliding parts and where the pads run, and reassemble.
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!