Have you looked at Softros Lan Messenger?
It uses LAN broadcast, no server needed. for 250 users it is $10/user and you're buying the software with 1yr support thrown in. After that support is optional. We let ours expire 3 years ago as we used it maybe twice.
We use Softros LAN messenger. It is super simple to setup.
Only 2 problems we've had:
Users are able to easily change the alias that shows up on the list.
If you clone with the same profile, it is possible to get ID conflicts.. which are easy to resolve by either removing general.ini before cloning, or changing an 8 digit number in the ini file after cloning.