party for 100 - what to feed them?

My record was about 70. I always pulled it off by focusing on a variety of finger foods, which could be prepared several days ahead. (If you are interested, I would be happy to email you some of my past menus.) But you will still have to plate all that food. And replenish the plates. And pour drinks. At least outsource that part.

I would hire a caterer, period.

Keep it simple. My family regularly does pig roasts (cochon d'laits) once or twice a year. Historically, it's been a potluck, with each family bringing a traditional dish. One year, we hired a caterer. And quickly decided to go back to the potluck style. One family buys the pig(s) (one 30 pound pig for 50 ppl; 2 for 100) and the "men" put them on about 6 am. The traditional sides: baked sweet potatoes in their jackets; dirty rice; coleslaw; potato salad; maybe a green salad. Other sides that sometimes show up: baked beans, texas caviar, broc w/ cheese, and cornbread dressing (instead of dirty rice). The more sides, the less of each you will need.

Hiring helpers is something they try to do, too, even if it is only to hire a babysitter to free the moms. Unless you can hire real help, use paper and plastic!

Keep it simple and seasonal. If you have a good authentic bbq restaurant with large smoking ovens I would choose maybe two protein choices between brisket, chickens, ribs, pulled pork.
Make a really good cole slaw, sliced summer tomatoes, summer corn(cut off cob with salt and butter heated just before party) doctor some baked beans, cold watermelon, tub of your cities best local ice cream

We have a lot of big parties and we always have tacos. Make the taco meat in advance and pit in a roaster. We have a tortilla factory where we got bulk taco shells and chips. Sam's club in a good place to find bulk shredded lettuce and cheese.

Another vote for outsourcing as much as possible!

100 people is a LOT of folks. If you want to enjoy your party, I would recommend going with not only ordering the roast pig, but also engaging the caterer! OR you could contact your local college which likely has a student employment agency to hire servers and cleanup-ers. Even if you choose to do the cooking yourself.
{Advice from a former caterer...}

Should read "parm" not Park. Auto correct issues.

A recipe I have made ever since the baby eggplants have arrived is this eggplant parmesan from Jennie Perillo: http://www.injennieskitchen.com/2012/08/jennifer-perillo-fairytale-eggplant-parmesan/
I have made it for up to 30, but I think it is very easy to replicate for 100. You can make the day before and briefly reheat. Four ingredients - any kind of smally eggplant that can be sliced in half (does not need to be Fairytale), marinara sauce (for 30 I used cans of Trader Joe's marinara sauce), olive oil to coat the baking dishes, and cheese for topping (I mix mozzarella with park or an Italian mixture).
Twenty five minutes in the oven, and reheat. You can make the dish days in advance.

Hum, I did a themed menu once with coconut shrimp appetizer, honey glazed ribs, chicken will a killer sweet and sour sauce, pineapple rice salad. A nice Asian slaw would have fit right in. Are you going to have refrigerator space for sides for 100 so you can do things ahead/chill things?

If you go with the pork you are going to have to kill and roast a lot of pigs to feed 100. For large groups I would suggest old school all the way. Pigs in a blanket (ha!), ramaki, cocktail shrimp, hand rolled sushi or gimbap (with tofu for the non-meatarians and gluten-free narcissists). For new school hand food, kosher hot dogs with kimchi.

100 people! Wow! Yes, order the pig. That is a fabulous idea. It would be way too stressful for a non professional to safely handle meat for the many and you'd have trouble judging the timing. Also, I hope you have a lot of help! You might consider trying to hire some young adults (like local college kids if possible) to manage the drinks and clean up as people discard dirty dishes. They can also help get the food out on time.
I have a hard time imagining the amount of food for 100....the most I have done is 30 and that we a lot of work. I would suggest dishes that can safely be served at room temp because oven space will be tricky if this is at your home. Ratatouille is very seasonal for September, delicious when cooked the day before, and can be left unheated.
If you have a local bakery, you can order a large number of baguettes to cut in pieces.

A huge salad.....I would do a simple green salad, tossed at the last minute, or else a cole slaw, because that can be prepared ahead.
Sounds like an adventure!