first of all, get a second opinion at a place that specializes in MB's. and then if the diags are confirmed consider the following:
-you should probably address the head gasket leak first.
-the tranny cooler lines will leak if they don't have the right seals in place which is common specially if the tranny has been serviced. most of it will wind up under the engine in the bellhousing area which could be what you are seeing. telltale is the color and consistency of the fluid. if it's red you know what it is. my tranny leaked at the two cooler pipe fittings and my mechanic replaced the seals for free (but then he had already done the head gasket job for me).
-the rear main seal sounds like it can wait if that's really what it is. you can keep adding oil until the leak gets really bad. you can add oil stop leak additives that will expand the seal, just check that it doesn't have toluene because that only makes it worse after a while.
-do the wheel balancing as someone suggested.
and after all this, so you're out a couple of thousand bucks. it's still a benz and it could last you another 100K miles with the proper care and attention. that second opinion will help you decide what to do...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"