Cylinder head gasket leaks on the drivers side do cause considerable problems and often are mistaken for rear main oil seal leaks. Of course valve covers gaskets will do the same.
Mileage will determine the logical way to approach such problems. If the car has a single chain and a gob of miles 180k or above, logic says that one should do a complete valve job and chain update with the head gasket. The complete valve job will usually run 25-30 hours with normal chain replacement. On the SL or SLC the solid pan requires removal of the subframe for pan removal. BTW here is a good article about chain removal: http://www.babcox.com/editorial/ic/ic40024.htm
We usually do the whole thing in the car and charge 40 hours for valve job and chain update requiring subframe removal.
Just doing a rear main seal requires removing the trans (about 7 hours in a 107) and flywheel. One should probably reseal the front pump at the same time which requires pulling the front housing. This will add another 3 hours.