potatoes, let's rethink these. You can make either mashed or scalloped without peeling the potatoes- it's trendy- try these at home first. Mashed is more difficult to reheat. Scalloped also acts as an entree for non-meat eaters. 45 pounds potatoes per 100. They could be cooked in roasters day of.
12 quarts of the casserole, about five 9- by 13-inch casserole dishes, fills a roaster insert and takes about 25 pounds of potatoes. Serves about 60. 350 degrees, 1 1/2 hours with hot sauce, up to 3 hours if sauce was cold, then turn down to 180. You can spray and foil-line the insert to ease clean up. They do better with white/cheese sauce than just layered with flour, etc and milk poured over.

green beans, take a look at the ellen's better
green bean casserole, link in the holiday cooking section at the top of the big pots page. Your proposed method would result in overcooked beans and undercooked potatoes
cole slaw, 7 gallons
baked beans, 8 gallons, have some meatless for the non-meat eaters, you will have some.

corn, 1 piece per person, or 24 pounds per 100

watermelon cubed with cut strawberries, 1 quart strawberries per 10, 1 pound whole melon per 4
homemade cookies, make smallish- 3-4 bites, 4 per person

chips, 1 pound per 12, 1 pint dip or salsa per pound
pretzels, 6 pounds per 100
vegetable trays use the veggie tray page, Ranch dip
I would definitely add cheese and crackers, 6 pounds crackers and 8 pounds cheese per 100

homemade lemonade, 6 gallons per 100
unsweetened tea, 8 gallons per 100, there are some great flavors out now
bottled water, 2 per person
beer, full keg serves 40-50 beer drinking guests