1. The head is the upper part of the engine.. the block and stuff having to do with the crank is the lower...they meant ' lower half'.
2. The rack damper bolt is provided to allow the setting of the idle to reduce shaking of the engine at some rpm's.....
3. Throttle linkage travel is simply the position of the metal linkage which connects the throttle pedal you push with your foot... to the fuel increasing/decreasing system of the engine ( maybe the governor or fuel injection pump ) compared to the position when pushed all the way in.... this has been the subject of a lot of people finding that when they checked/corrected the travel available they had LOTS MORE POWER... a 20 something year old car can get worn/out of adjustment slowly....and no one expects a diesel to have much power anyway... so it slowly gets to where it has like only 3/4ths of the travel it should....
4. THe thermostat is a physical device in the coolant stream which directs water to the radiator in varying amounts depending on the need of the engine for more or less cooling..
The Guage sending unit is the thermal reacting item screwed into the side of the block which generates electricity in varying amounts which the guage in your dash sees and interprets as different temperatures...
6. No that is not a problem..
5. You really need to address a couple of things if you have a rough idle... check the Tech service bulletins which have been issued for your car by looking at alldata.com
and find the directions for setting the rack dampner...
and read up on setting /checking the engine mounts...

Ps, I think you would really enjoy having the paper factory shop manuals for your car... these are complicated and " over the top' designed cars... I am regularly surprised at how MB did something compared to all the other car makers in the world...

also, the search feature on this site really rocks.... and when something has already been posted you don't have to wait and hope someone will stop by and know the answer....