Your mechanic buddy is just being thorough. Changing the seals is never a bad idea, and the odds of wiping out the cover gasket are pretty good given your bike's age. Better to have one on hand, and since you're ordering one, might as well get the seals.
I do think he's mistaken regarding the chain rubguard, though. The portion that runs on the top of the swingarm should be about 2 inches longer than the lower part. The lower portion is only about an inch long and squared off as you describe.
One trick that works fairly well for me is to glue the gasket to the COVER side with a VERY light coat of Permatex non-hardening Form-a-Gasket, then lighty oil or Nevrsieze the engine side of the gasket. 9 times out of 10 I can reuse the gaskets if I lightly tap the cover with a rubber mallet until the cover comes free on its own, rather than forcing or pulling the cover off.