How is your oil consumption? After how many miles do you have to add a quart(if at all)? I would say if your oil consumprion is low then your cat would not get damaged.
Do a search on this site for do it yourself valve stem seal replacement on your engine and check the Fastlane for tools and parts.
I have the same problem with a 300d and found directions for valve stem replacement on this site. There is a Haynes manual for your vehicle available. Or you may want to slurge for the MB manual on CD.
'00 Mazda MPV Family's car
'83 300D 235,000
'84 109E 175,000
'74 280C 125,000 in garage; needs rehab or sell for parts???