Thank you guys,
I am also concerned about safety and never had to deal with a caliper seal. As luck would have it, the local MB dealer had the kit that Haasman mentioned on hand. I have a good relationship with the parts guys and one of them offered to put it on for me if they couldn't get a mechanic to do it for me this afternoon.
So I took the caliper to them and paid the $32 incl. taxes for the kit and he installed it for me. He wouldn't even let me pay him for the labor.
I put it back on after cleaning the parking brakes and then bled the system. My short test drive this evening indicates no trouble so far.
But I know now I have to deal with the other 3 calipers too, as that rubber is just as old although not torn.