here is a diy article from this site.
http://www.peachparts.com/Wikka/W124BrakePads
Changing the front discs is easy once you have the pads out and pistons pushed back, remove the 2 bolts ont eh back of the caliper that hold it to the car, lift the caliper our of the way (don't let it dangle from the brakeline)
Undoe the little allen screw on the rotor and remove it. put the new rotor and put the screw back in, then put the caliper back and finish up by following the rest of the DIY article.
Rears are similar process, only you have to also be careful with the parking brake mechanism on the rear.
I can send you the pdf from the w124 service manual if you like on the front and rear brakes too. PM me if you want me to.
Alon