The ticket is to serve as many foods as possible that can be prepared ahead of time. I don't know if I'd do mashed potatoes, I'm not that ambitious.

The more you can do ahead, the more fun you'll have at the party.
We just had 55 for an engagement party. This is the menu I used for this many people. It's the same menu we used to use for our annual Ag Society appreciation supper, and it's always proved to be a good meal with not much fancy work. Two in the kitchen could feed a hundred.
Beef. Of Course. - we used about a half pound per person.
Potato salad - four gallons. make the day before.
Coleslaw - two gallons - make the day before.
Buns - one and a half per person - buy them. :wink:
Pork and beans - My son calls these Ag Society beans. Fry up a pound of bacon and a large chopped onion. Add canned pork and beans, all varieties, however many cans you think you may need. Throw in a big handful of brown sugar and a big glob of ketchup, and fill up some slow cookers. I did the bacon and onions the day before, then just tossed the rest together the morning of party. When I made the beans this time, my son forgot to bring his slow cooker, so I had a panic attack. I ended up putting the beans all in a big soup pot and put it in the barbeque over indirect heat. They turned out awsome! They picked up the smokey flavour from the barbeque.
Taco salad - Cook up a couple of pounds of hamburger with taco seasoning the day before. Just before the party, put a couple of bags of mixed salad greens in a big bowl, crush a bag of Nacho Cheese corn chips over it, shred a pile of cheese on top and some sliced onions, then pour either Russian or Catalina dressing on it. Dump the meat mixture on top and toss it.
I also made a couple of pasta salads, but they're pretty simple. Just cooked pasta with store bought salad dressing, whatever flavour, and some onions, celery, and assorted whatevers in it. Day before.
Have a great time! And don't work too hard.