I have own M-B wagons for over 20 years now. I just love them, eventhough they tend to be a little delicate in certain issues as all of you have mentioned before.
The rear leak on the side windows is typical and can be easily fix by changing the rear side window rubber seals. The secret though, is in HOW the rubber seal is glued back. The dealer tells me that a special glue is necessary to fix the problem. If you use the regular glue everyone uses, the problem will come back sooner or later.
The non-working speakers at the back can also be traced to a common W124T assembly line design flaw. My 1995 E320T came brand new with the rear left door speaker totally mute. The dealer opened the B-pillar moulding, and as suspected, the wiring was cut by it. They replace the all the wires and finally had to change a fuse in the radio. When the wire gets cut by the moulding, it grounds the wire and the fuse inside the radio blows.
Finally, the stucked rear seatback is also a common W124T design flaw. The auto mechanism which automatically releases the locks on the seatbacks as you open the rear doors tend to fail and the the rear cargo area floor panels have to be dismanteled to access the circuit and fix it. With my W123 300TD wagon I never had that problem, as it lacked the complicated release system.
Other typical issues with "T"s is water filtration through the windshield. This usually floods your interior in minutes when it rains hard. Some hydroneumatic systems in the rear self-levelling suspension tend to be a constant headache to some owners. And last, but not least, the rattle concert you must endure if something is loose or not properly placed in the cargo area side compartments.