I'm not sure what a OM603 engine is. Is that the engine in a '87300SDL? I got one about a year ago but haven't worked on it yet because I haven't used it much.
However I would not change any of the GP just because they were old or I no records on them. When and if they go bad I will replace them. I always carry a spare GP like I used to carry a spare spark plug when I drove gasoline powered cars.
Even if a GP goes bad, it isn't a show stopper. One cylinder will just miss-fire for a short time after starting and then the engine will run normal until it is started cold again. The bad GP can then be changed at my leisure.
P E H