OK, being a cooking aunt myself, I can see the possibility.
Consider the option of doing meatball subs instead of sloppy joes. More interesting, you can even buy the meatballs readymade and frozen, and can be served on sub or hoagie rolls which are both tastier and prettier than hamburger buns.
1 pound of raw burger makes 1 3/4 cups cooked. Allowing for a few seconds, this is 6 sandwiches- I would plan 360 sandwiches, or 60-70 pounds raw beef. 360 buns/rolls, of course.
See the sandwich event page for the condiment table per 100 for the fixings for the sandwiches.
I would add slaw- easy to fix and hold, well liked- 60 pounds.
Baked beans, 3 gallons or 4 #10 cans per 100 people, plus your add-ins/ fixings. Or 8 pounds dried beans per 100 from scratch.
Potato salad is right.
You need some things out as appetizers during the hour you are getting the wedding photos, changing clothes, etc. If you/ your aunt will look at the veggie tray page, you will see amounts for the veggies and dips- you do need dips- and you will hae a much better looking tray if you add broccoli bits, cauliflower, some olives and pickles, etc. This can all be prepped several days ahead- use your chips here, too, 6 pounds potato or 8 corn plus about 6 quarts dip or salsa for each 100.
