I am not big on additives, but there are several threads on this BB that recommned a waterpump lubricant from Redline called Wetter. I beleive it is sold by this site in the PartsShop.
I am not a 190 expert. Usually these are very easy to change, so Ipersonally would not bother with the thermostat, but others might disagree. It is easy to change during this work or at any other time. If the 190 thermostat is unusually hard to get to, you might want to reconsider.
I would recommend changing the "fan" belt(s) while you have all the radiator stuff out of the way. AT 115K, it is due. If it has already been changed, consider doing it again, especially if they have accumulated 30-50K. I am not sure how the 190 is set up, but my 87 300TDT has a serpentine belt. If it is like mine, the belt tensioner should be changed as well (in my opinion), since the belt tensioner pulley/bearings tend to wear out. You might want to put a post up with this specific question if no one jumps in on this thread.
What about the odor in the car? I would still suggest a pressure test to confirm/deny the need to change the heater core.
Lastly you say you are new to this, so I will give you my safety speech:
Antifreeze is taste sweet and deadly to animals and children.
Let the cooling system cool and depressurize before opening the system. It can bite you!
Working under a car can kill you! Buy good ramps, jack stands, floor jacks and work on stable ground (concrete). Set brakes, chock wheels, and have an assistant around at all times that knows how to use the jack to exstricate you should the car fall, roll on top of you. Sounds impossible, but it happens ALOT. The money you save doing it yourself will easily pay for this equipment.