X.X.
Pasta! Easy to prepare in bulk, easy to reheat, and cost efficient. Baked Ziti in chafing pans? Ziti noodles, a ton of sauce, cooked sausage stirred in, and lots of cheese over the top. Leftovers freeze and reheat very well.
My other idea is taco bar. All the fixings and a hot pan of taco meat and pulled chicken. A little messy, though, for a formal affair.
For the large gatherings I've cooked for, I have googled "Cooking for 50 People" and there are lots of gals who have done the work for us. You'll be able to figure out how much to make per person.