For a handful of servers, try zabbix. Every distribution has a packaged zabbix agent. It has everything: web ui, a way to Auto discover things with a bit of setup, nice graphs, alerting, LDAP User Management if you need it, a way to define per person/group alerting/notification schedules. And the community is big enough that many common services (fail2ban/postfix/MySQL/etc.) have premade custom monitoring scripts. Adding your own metrics is also very easy.
How old are you?