You can change the plugs with the manifold in place but it's a nightmare. I'd pull the manifold. No need to remove the injection lines - leave them attached to the injectors and injection pump. Replace ALL SIX plugs... AutoZone has the best price I know of, and they have Bosch Duraterm plugs. Use Bosch or Beru plugs only - not Monark, Autolite, or anything else. While the manifold is off, use the time to clean things up, use a new OE/dealer manifold gasket if possible (this will have a metal reinforced core, most of the OEM/aftemarket gaskets are paper only). Also replace all the metal injection line clips & buffers - see photos below.
As a 1987 300D owner, read
this article if you haven't seen it yet:
