Why not call some sound places and look into renting?
None of this makes sense... If you don't own the theater, why would you want to buy speakers to fill it for a film festival that is only going on for a short time? How are you handling the projection for a 200 seat theater? If they can provide a adequate projector, then why not have them provide adequate sound?
It seems like BUYING something is just not the way to do this setup. Checking the used market, hard, would be a good concept, and borrowing wouldn't be the worst thing, but rental staging would be my top recommendation. It may get you into a decent system for the duration of the festival, without costing you the $10,000+ it should run you to do this properly.