Pork butt would probably be the easiest to deal with, it's pretty foolproof. I'd figure 1/3 pound of pork per person, if you're planning on having sides. That would work out to about 66 pounds of uncooked meat (figure on a 50% loss) for 100 people.
Deep South GC36
Lang 84 Deluxe
GMG Jim Bowie
Yes, Dear BBQ on Facebook
Proudly sponsored by Dixie Meat Rub, Baxter's Original Smoking Wood, and Deep South Smokers