Welcome to the Forum and congratulations on your purchase.
Sounds like you may have made a good deal if the Head Gasket diagnosis was a false alarm (I certainly wouldn't want to suggest that the "problem" was created just to encourage the previous owner to buy another car
).
Keep a close eye on the Temperature Gauge - your readings are perfectly normal right now. After using the car and leaving it overnight, check the Top Rad Hose in the morning. If it is soft and pliable that is fine. If it is hard as a rock, then you may have a potential Head Gasket or (worse) cracked head.
Check for oil in the coolant and coolant in the oil.
CHANGE THAT OIL NOW with a good 15W-40 Diesel rated oil. If your present oil is just black, that is normal with a Diesel. New oil will turn black after only a few miles.
Change the Oil Filter and the two O-Rings on the Filter Stem - not included with the filter but about 10 cents each from a dealer.
Change all your other filters unless you know they were done recently - Air Filter and both Fuel Filters.
If in doubt about when it was last done, Flush the Cooling System and re-fill with M-B Coolant or Zerex only (should be pink not green).
Check the condition of the Drive Belt and Tensioner. The Tensioner is a known weakness that needs replacing from time to time.
Look at the Vacuum Pump (front of engine below Power Steering Pump). If it has a smooth front cover it is an original. If it has four screws on the cover then it is the updated design. The original pumps if they fail can cause catastrophic failure of the engine so most people replace them as preventative maintenance.
Check all the fuses for any corrosion and replace any that appear bad. Flaky Fuses are the source of many electrical gremlins.
Good luck, these are GREAT cars and properly maintained will last a loooong time.
