That is the serpentine belt tensioner, right in front. OM603 engines tend to lose that item regularly and the result can be a busted timing belt (resulting loss of water pump and alternator all at once and the engine will be in dire straights.)
Presuming you go ahead to replace the head gasket, I suggest you plan on having the head sent to a head shop to check for cracks in the critical areas where the early heads sometime fail. And they will perform a skim cut to make certain the head is perfectly flat. If there are cracks or corrosion spots that are too deep for a skim cut you will possibly need a new head, just be aware of that.
As long as it is off the engine you should have it checked at least.
I lucked out with a rebuild on my '87 300D turbo with #14 s/n head at 250K miles there were only a couple of small corrosion areas, nothing major, and the head was flat within a few mils before surfacing. A new head gasket and it is back in fine running condition, no more leaks.
The cam on these engines can be broken that is a known issue. The lifters apply a lot of force on the cam and if one goes to remove it without following the proper sequence the cam will break.
DDH