For the baked potatoes, you need 1 per person. Wholesale produce places sell them bu the 40 pound case sorted by size, you can get these from large to humongous. Much nicer than bag potatoes from the supermarket and cheaper than the loose ones.

For the beef, 40 pounds raw boneless, and someone should carve and serve.

Two suggestions. Have a simple appetizer table, punch and 3 items for the hour while you get photos done after the service and that also gives the kitcheners toime to set up the buffet nicely.