I second the Nagios recommendation. It may be an eighteen-wheeler, but to set it up with a small config like this is very quick.

On the other hand, since ssh access is already set up, the nagios check plugins can be installed on the remote server without bothering to set up a full blown Nagios server. Then just use a script to ssh in and run one of the nagios checks and take action based on the exit code of the plugin.