88 300-TE uses a Bosch plug number -> H9DC or H9DC0. Either will work fine. The H9DCO has a slightly wider side electrode, but in my experience, the H9DC seems to idle smother.
In another recent thread here, some of us learned that Bosch no longer makes these plugs. I suspect there's still a surplus available at the various WEB seller sites like the one here or at a local euro retailer in your area. Once that supply dries up, your options are Beru plugs which are available at:
http://www.kingsborne.com
or Asian brands like NGK or Denso.
Here's a photo of the H9DCs from this sites BuyParts facility:
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1U40N9POF1U40N9W6S&year=1988&make=MB&model=300-TE-001&category=F&part=Spark+Plug
The Bosch replacement for the H9DC is a resistor plug. Unfortunately the H9DC and H9DCO are non-resistor. I'm not sure what Bosch was thinking about when they did this?
Here's a pointer to the H9DC discussion that went on awhile back:
H9DC and H9DCO spark plugs Discontinued