What sort of business is doing the open house?
If it is something that does lots of business on the day your holding it, does he have any numbers for that day of the week, walk ins etc.
The more information he can give you the better.
If he says he gets 500 through the doors, don't cater this time for more than 50% of them, then build off the numbers for the next one.
The idea about pre-packaged and re-heated is a good way too, you don't have to take long to reheat, just make sure you get it all above 165 for 30 seconds minimum, to help keep it safe.
Because of the yield difference on pulled pork and brisket, if you want 200 sammies and say 100 of each, you'll need 67 lbs uncooked butts, and 56 lbs uncooked brisket.
"There is no such thing as a little garlic." A.Baer