Congratulations on the new 300SD. You have bought one of the best cars Mercedes ever made. Was this the blue one you were looking at or a different one?
As a former 300SD owner:
1) Use Mercedes Benz anti-freeze or Prestone made for aluminum radiators, it is orange in color. This will prevent any phosphate build up. Do not use regular anti-freeze and make sure you have the cooling system flushed.
2) Engine oil: Castrol 20-50W or any other good brand is fine. I have always used 20-50W here in Southern California and my car had 287000 miles on its original engine when I sold it.
3) If this was the one that needed shocks, make sure you put Bilsteins, "comfort" not HD. The ride will be too stiff.
Let us know if you have any more questions.
1987 Jaguar XJ6