I completely understand their not wanting to get into something this old. "We've" entered an era where customer's (I'm not saying you're this way) have become very difficult. So shops are learning to cull those folks, by saying what you're being told.
It's been a long time since I was R&Ring placing automatic transmissions on MBZ, BMW, etc. so I'll offer a few suggestions. Just bare in mind that I'm not the god of this stuff, and it's been a while for me.
I can't tell by your post, but it appears that you've removed the transmission, or someone else has removed it for you? If so, when the transmission was removed, did the TC get left on the flex plate (flywheel)? Or was the TC unbolted from the flex plate, and left on the transmission, securing it in place with wire, and then the transmission removed from the car?
Suggestions:
Lift the TC just slightly, and slide it off the input shaft in the transmission.
You will now see the TC seal (input shaft seal), and the area of the pump flange, etc.
Start by looking for the cleanest area of the leak zone, as mentioned. This will be your leak source.
If the TC seal looks like it hasn't been leaking, you can test it for how flexible it still is, by taking a small, thin screwdriver, and gently pushing on the seal's inner lip. If it seems to be really nice and flexible, and not cooked and stiff, that seal is probably still good. You can take a rag and clean out the seal lip grooves, and re-grease it. If the seal looks like it's been leaking, pry it out and replace it. If you can't find the correct part number, the seal can be measured and replaced based on it's size. It may have a part number stamped on it, or it may have it's metric size stamped on it (outside diameter, inside diameter, and thickness). If you can't get the OEM seal, try to replace with one that's of a higher heat rated material, so that it won't get cooked hard by the hot fluid, and the heat of the TCl. The cooking of the seals is the death of all automatics, whether an external seal like this, or the internal lip seals and o-rings. That's why it's important to keep automatic transmissions as cool as possible, especially in hot climates, or hilly conditions, or when towing. When the seals get cooked hard, and leak, the internal clutch packs stop getting sufficient hydraulic pressure to stay clamped tight. The clutch packs then start to slip, which generates more heat, and frags the transmission with clutch friction disc, and metal plate debris...destroying the transmission very quickly.
Enough about the TC seal.
If you think the pump cover seal is leaking (gasket? o-ring?) and it looks like the cover can easily be removed, then definitely replace it.
When it's time to put the TC back on, you have to be really careful about not damaging the TC seal. The splines inside the TC opening need to be centered in the opening, and kept that way during install. The TC is gently slid on, and slightly rocked and rotated left and right, so that the splines all have a chance to move around and mate up, while you apply "some" installation pressure, to get the TC to drop into position fully. You than pass wire, from one side of the bell housing to the other, over the front of the TC, in order to trap the TC against the transmission, so it can't slide forward, as I mentioned previously. Sometimes the TC is easier to install with the transmission standing upright, and the TC dropped down onto the input shaft. You can use the weight of the TC to help it fall down into place, but you also do a gentle lifting and rotating motion to get those splines to line up, and then you let the TC come downward and see if it will drop on it's own. If it doesn't, you gently lift a little, and keep rocking it left and right, and then letting it drop again. Once the splines are all lined up, it will drop down into place really easily.
Hopefully others who have much more experience/current experience, with chime in, and really help you out.
