You already have a syslog server. One is built-in to every version of Mac OS X. It's the system that builds the standard system logs (well, technically, it's not syslog, per se. since it's an Apple-developed log server, but it follows the syslog standards, so for all intents and purposes, it counts).
The main caveat is that the log daemon, by default, only accepts log messages from the machine itself, not network clients. You'll need to edit the configuration to support network logging.
The logging process is managed by launchd, and it's configuration file is at /System/Library/LaunchDaemons/com.apple.syslogd.plist
If you check this file you'll see a commented section that talks about network logging:
<!--
Un-comment the following lines to enable the network syslog protocol listener.
-->
<!--
<key>NetworkListener</key>
<dict>
<key>SockServiceName</key>
<string>syslog</string>
<key>SockType</key>
<string>dgram</string>
</dict>
-->
Uncomment this section to enable the network listener, then just point your firewall to log to your server's IP address.