Considering we had a server crash lately we really need a monitoring solution that covers the web and build servers
So I took a look at icinga2, easy to set up and the icingaweb2 really turned out nice - but not that great regarding visualization and check plugin setup (still too new I guess). I'll take a look at check_mk+nagios next
So after looking at Nagios and Check_MK as well, I went with Icinga2 after all which is a bit more straight forward regarding the configuration and has a passive agent built in. Icingaweb2 does all we need for now as well.