Please check the link
http://www.left4dead411.com/forums/index.php?action=vthread&forum=9&topic=8755
Has great plugins and mods that allows the versus servers to stay stable, also there are some admin tools incase you want to have admins for the game at a later stage.
Peace
A quote from that link
b. Get this plugin: L4D Director Enforcer This plugin forces the server to versus only and keeps the server from reverting back to co-op. Also, it allows you to change the difficulty for versus mode, thus unlocking it out of normal mode. You can choose to keep normal mode though. DDR Khat was able to implement a voting menu though for difficulty in versus. So, even setting this difficulty still means players have the freedom to choose whatever difficulty they want!
