Nice find! I've rehabbed 3 or 4 vintage
coach briefcases, none as old as your Dad's, but they all came out pretty well.
Considering the condition that it's in, I'd give it a bath and a scrubbing, stuff it and shape it, condition it and finish with Black Rocks, and then polish the hardware.
As an example, below are before and after pics of a black Lexington Briefcase that I rehabbed. It's from the 1990's so not as old as your bag but it was in pretty bad shape and it was improved by rehabbing IMO. Good luck!
Lexington Brief 5265 Before:
https://forum.purseblog.com/threads/coach-rehab-and-rescue-club.833400/page-970#post-31594799
Lexington Brief 5265 After:
https://forum.purseblog.com/threads/coach-rehab-and-rescue-club.833400/page-970#post-31594837