Mercedes doesn't even include a reamer in the WIS instructions for replacing glow plugs.
The process is, in a nutshell, remove intake, remove glow plug, install new glow plug, reinstall intake. DUH!
There is, however, a special "Glow Plug Repair Set" for "removing broken off glow plugs out of cylinder head as well as thread repair," part#: 611 589 00 99 00.
Includes about 20 various bits, taps, depth stops, thread cutter...."Special Order, Price on Request" from Samstag Sales.