Generally, when the valve seals on these go bad, the valve guides are not far behind.
Replacing the heads on these engines is not trivial and rebuilding them is not that easy. Even if you pulled them and had the heads done, there are a lot of "while you are in there" parts that can easily add up to (another) high three figures. Most of the rubber and vacuum lines will not survive being disturbed while the heads are removed. Then, there is the issue of helicoiling (actually, timeserting) the block because the screws holding down the heads screw into the raw aluminum in the block. Sometimes, you can get away without it; sometimes, you might be tightening the last bolt, you can't get to torque, and now you have to do the inserts anyway AND you ruined a $60-$90 head gasket. I've done the time-serts with the required jig (which is a fairly rare and expensive tool) and I don't see how anyone could do this while the engine is in the car.
The W126 forum is worth looking at and searching through because they have a lot of hard-core engine guys on there. One guy with a very respected shop trusts only a very small number of machine shops because of longevity issues. Anyone can do a valve job. Far fewer can do a valve job that lasts 150K miles.