I hear ya..
You are running into the few Bads of the E320..
Might as well throw in Head Gasket/lip seal, while we are at it..
But , that is the Nature of the Beast....
Aside from these , these are pretty good cars..
The OVP and plug connectors are no big deal, but the others sure are..
The line that plugs on the EGR is the exhaust gas return line that goes from the egr , back behind the engine , over to the intake side ..
Look up SB article in the DIYer section. There is a pic there..
Some are actually having LUCK with Benz picking up the tab , as there was a Recall on the egr..[ email me off line and I can send it to you --you can then see if yours is in the "Campaign" [ love their use of that over recall ] number series....
It seems that once any of these are taken care off , re-ocurrence does not seem to be a problem.
On the harness , it's a skill level thing , but not really hard --plug in , routing kind of stuff..