Guys,
I have a '87 300E with 200K and a '95 E420 with 108K. They both are registered in the same month (lucky me). Both required smog checks. I took them one at a time to my indie. He leans out the mixture, takes the car to the smog check, gets it passed, returns the mixture to normal and gives me the paperwork. Costs about $100. Both cars have the original cat muffler. The O2 sensors have about 30 and 50K respectively.
I wouldn't think about going it alone and trhrowing parts at your car. Let your mechanic handle it.

Anziani