Check out this thread which is similar to your problem with oil consumption:
cost of valve stem seals vs. do-it-yourself on '89 300E?
I've noticed that stevebfl, one of the moderators, mentions this everytime the question comes up about oil consumption and valve seal / stem replacement. He is utterly convinced that most of us are changing valve stems unnecessarily. The head has to come off to do this and thus very expensive (about $2,000 job with seals, stems, and head gasket). In his experience, he only sees the need to replace valve seals, which a DIY'er can do very very cheaply, or a shop can do it for 4 hours labor, approx. $240-$300 in labor depending on shop rates.
Do the valve seals and enjoy the car for another 150,000 miles. Stevebfl says the car will be good well into the 300,000 mile range.