For pulled pork, you are fine. doing a three load run on butts will more than suffice for 100 people.
My advice on stepping up.
1. Try and get a handle on the composition of the crowd, in terms of adults/children and male/female. I like to figure on cooked amounts and back out. For most cooks, men will be figured at 1/2 pound cooked, women at 1/3 pound cooked. Unless there are upgraded sides.
2. What is your menu. Once you have the crowd sized, the next thing you need to know is what is the menu. That controls a lot of your costs. Pulled pork, then work the menu from there, bread, sides, sauces, etc...I always figure servings by weight, served. Then back out for waste and loss during cooking.
3. Number three, what are the expectations in terms of tables, chairs, settings, utensils etc...nothing like thinking you are serving food, only to find that the host forgot the plates. Make sure everyone is on paper, understanding what they are supposed to be bringing.
4. Location matters, can you drive up, do you have to ferry stuff in, will you need a helper or extra cook. All of this matters, if you have to go up stairs, or down a lawn, that all adds time, which is money.
5. Once you have all your food costs, look at your estimate, it should be 3x food costs, plus, if you are providing hard goods, that costs 2x at least.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."