I have changed glow plugs on a W124 with a 603 engine. I can not image that trying to change the glow plugs with the intake in place is worth it. It takes relatively little time to remove it. Comparing glow plug replacement on a 606 verses a 603 I think that the worst thing on the 603 is all of the little brackets that are attached with the manifold. When the 606 was designed all of the was removed. Also, when you remove the intake you can clean it out.

Purchase the glow plugs and manifold gasket, drive to Ohio, and I will replace them for you and you can watch.