I know this is an old post but I am just now resolving the same problem on our 1972 Mercedes 280SEL 4.5, which had stuck calipers from lack of use even though the car itself is in beautiful condition and still owned by the original family. It turns out that to not disassemble the caliper halves is an Internet myth and indeed it is important to disassemble any caliper to properly re-plate (yellow zinc, then paint any color you want over the plating) and restore it.

It is correct that the seals are not available through Mercedes but they are available through industrial sources as they are rather basic Ethylene-propylene (EPDM) square cross section o-rings. I am still trying to determine the exact size but they are something like 9.5mm OD; 4.75mm ID and are about 1.5mm thick. That’s about 3/8” OD; 3/16” ID and 1/16” thick. Since my own have been crushed inside the calipers since 1971 or 1972, I imagine the thickness should be a bit more.

I hope this help and if I learn more I'll post it.