Yes, the OM603.96x is the 3.0L turbo engine used in 86-87 300SDL's and 87 300D's. You are correct about the GP operation not causing anything really bad when one fails, other than a very rough running engine for the first minute of a cold start and a cloud of smoke too.
The whole reason I mentioned it is simply because of the difficulty of replacing them one at a time, only when they fail. On this engine, they are simply awful to replace. It's not like the older (through '85) diesels, where you have pretty decent access to them.
Regardless of engine, though, I believe they should be replaced as complete sets (except perhaps for freak short-term failures) - and more so on the 603 due to the increased labor time. If the plugs are original, or very old, and one fails - it's a bear of a job, and you'll probably end up doing the other 5 one at a time over the next few months or year or whatever. Why do an unpleasant job 6 times? My philosophy is that if one old one of a set fails, change them all - that way you do one unpleasant job one time. And, then you shouldn't have to worry about them for a LONG time, possibly the rest of the time you own the car!