What you recommend to monitor very old systems like Debian 5 or Ubuntu 6.06 up to 14.04?
In addition to checks such as http, ping and ssh, local tests like apt, disk, procs, load, mailq are supposed to run on every host.
Newer systems such as Ubuntu 18.04 and Windows Server 2016 are also in use and are currently being monitored through icinga2 agents and config sync. The configuration on the master is done via the Director module. About 500 hosts should be monitored.