120 pounds of mashed potatoes
66 pounds of corn
66 pounds of green beans
70 pounds of cole slaw

Is this combination ok? Too few/too many sides?

Also, is gravy necessary?

With 300 people you are going to have some who don't eat ham/ pork/ meat. You need to plan either a meatless entree choice (about 30 servings) r a side dish that can serve as an entree- mac and cheese, eggplant parm, etc-

Why You May Need an Appetizer Table
A self-catered buffet is a real challenge to the organizers, and things don't always come together as smoothly as hoped. By planning and setting up an appetizer area or "nibble table" which is ready when the guests begin to arrive, the cooks and servers have time to complete the arrangement of the dinner buffet. This is especially critical if there are children among the guests, or there is a wait before dinner, as at a wedding reception.

This does not have to be complicated. Three items, including a fruit tray, a cheese tray and an attractive dip or spread, plus a punch, or two if one is alcoholic, are sufficient.

One appetizer service area is needed for each 100 guests.