Lee, I think the tool you highlited may be the 6mm allen-head bit, which is used on one of the 4 water pump bolts (at least it is on my '91 300SE).
But that's the easiest one to get to, no big deal at all for a std allen wrench.
The rear two are the buggers. I just did mine for the 3rd time two weeks ago, so it's fresh in my memory. I did get to all 3 of the 13mm bolt heads with a variety of sockets, wiggle-tails, extensions, etc. It's no cakewalk, but I did it without removing anything but the belt tensioner, and loosening the power steering pump a bit. Think Masochistic Contorsionist, and you can do it.

DG