First of all, please go read my article on self catering your wedding reception in Big Pots, if you have not alreadt done so. I meal this size requires a professional/restaurant quality kitchen, 3 cooks and at LEAST a dozen servers, even if done buffet style.

The plan about the precooked beef is not a good plan for two reasons:
1) It is unsafe. See the discussion about turkey reheating in big pots. The broth over cold meat does not heat quickly enough or get hot enough to get above 140 degrees- they are not safe in terms of bacteria.
2) Buffet serving of the presliced thick standing rib pieces is VERY EXPENSIVE. You could have a carver/server and a catered hot steamship round for less than this would cost, much easier on you, also. Or many other beef dishes.

There are lots of potato dishes that can be made the day ahead. For this meal if you are going formal, I would probaly use the freezable holdable mashed potatoes or else twice-baked stuffed potatoes. Scalloped potatoes are delicious but less formal.