Have a look also to Icinga for host/service health monitoring and alerting. It has good integration with Graphite and a C style DSL for host checks configurations and it’s fully compatible with existing Nagios checks. Its functionality can also be extended with other tools via modules and plugins (eg. director, reporting etc).
Grafana+Prometheus is another good option. Prometheus has also builtin alerting functionality via Alertmanager which has to be configured separately though. Grafana has also pretty interesting “satellite” projects like Loki for log exploration.
My two cents for netbox. Pretty neat and simple DCIM tool with all the features you probably need to organize your infra information.
We have been running all of these on prem (single node setups) quite some time now without any major issues. Something that has to be considered carefully is the storage needs of Prometheus (if you would like to have long data retention policies) and that you need a separate project in order to have a scalable highly available underline storage for it like Cortex or Thanos. Additionally for Icinga, it has builtin HA functionality but it needs some searching and testing to make it work without issues with Icinga’s Web2 GUI.