I don't know your cooking arsenal fully but I think you have enough to pull that kind of cook off. I'd figure on 1/3 pound per person. I'd also add 10-15 more people to the total, more always show up. Then again, some may drop out so it may stay at 250. We'll just keep it 250 for the sake of keeping it "easy", lol.
1/3 pound X 250 people = 82.5 pounds of finished product. In my experience, you yield about 40-50% of a butt's pre-cook weight (if doing bone in butts). I am VERY stringent on what makes the cut though, some people take the entire butt and have at it bear claws or a mixer on a drill and mix the whole shebang up, I'm not that guy. I hand pull every strand of it and sort out the fat and gristle and unrendered fat. So with my ratios listed, I would cook about twenty 8 pound butts. If you have extra meat, like mentioned above you could donate it to a shelter or even to the local fire department or police department. I will tell you that the police are WAY more leery of accepting food vs. the firemen. Guests may even want to take leftovers home but I doubt it. Last case resort would be for you to take it and put the foodsaver to work and save it for yourself!
One other option, cook it ahead of time and pull it but minimally, keep it in larger chunks to retain as much moisture as possible. Food saver it and reheat the day of. That's A LOT of extra work and costs though, it'll take a BUNCH of Foodsaver bags, lol.
As for the sides, that's a tough one to figure out. I always make too much as I always have a huge variety. Keeping it to 2 things should help you figure it out better.
Something baffling to share, I cooked for a friend who "catered" a church event and someway, somehow, he fed 120+ people using 6 butts I smoked for him ranging from 3-4 pounds to 7-8 pounds pre-cook weight as well as 6 whole chickens I smoked in the KBQ (very first thing ever cooked in it) plus some potato salad and baked beans and he said he had leftover meat. Now either he pulled some Jesus with the bread and fish action or the food sucked. He said everyone loved it! I still can't figure out how that little amount of meat fed everyone. It's just not possible. It should've been enough pork for 54 people at 1/3 pound or 72 at 1/4 pound per person. The chickens were 5 pounders and the yield on that might be 2.5 pounds (each breast 8oz, legs thighs, wings, back might equal another 1.5 pounds). So 2.5 pounds x 6 = 15 pounds of chicken if every usable piece was pulled (I doubt it). 15 pounds serves 60 people at 1/4 pound per person. I know I am WAY over analysing this but it just baffles me. He had meat left over, didn't even pull meat from 2 or 3 of the butts, left them whole in the cooler. Had chicken left over too and told me people were piling their plates high, lol.
He injected them and rubbed them, I just cooked them. I can tell you that my smoker was FULL of a funky yellow congealed gook after smoking those butts! I don't know what he did to them but I wasn't liking what I saw. I feel he wanted to put his "touch" on it so he felt inclined to "flavor them up" as he calls it. He should've left that to me, lol. I am not used to people bringing me grocery bags full of leaking, injected meat to put into my fridge to wait for smoking