Leave the injectors alone unless you know they are causing problems. The quality of what you can buy now new is most likely less than what you have installed now with 178K miles on them. Yes, the new ones can be that bad.
I pulled the intake manifold and did a bunch of other work while I was changing out my glow plugs. There is a guy in Texas who said he did his without pulling the intake. I think he had an SDL which has more room in the engine compartment. Do the resistance and voltage checks described here before you do anything else with the glow plugs:
http://www.dieselgiant.com/glowplugrepair.htm
One good note, they rarely break off in the 603 engine. It is the newer (mid 90's) MB diesels that have that habit.
-Jim