There are different types of PBR brake pads for W123 Mercedes. One is called MetalMaster and it is still available.

As far as your ML goes, I don't blame you for trying ($30 vs. $100+), but we're basically afraid of PBRs for newer vehicles. Hopefully, Mintex will eventually produce rear pads for MLs -- that's what we keep waiting for. In the meantime, stick with the OE pads.