I've been working on Nagios for 6 year. I know well both Nagios Core and Nagios XI, i'm a system administrator on it and a system integrator. I'm able to install Nagios on a new fresh system, upgrade and manage it.
I can provide training almost on every feature offered by Nagios and i can help you from the installation to the configuration of Nagios (hosts, active and passive checks, smart management of them using service templates and service groups, contacts and notification management...). I can also suggest the most common wizards,plugin and tricks in order to speed up the creation of new services.
I worked a lot on Nagios and my experience comes from a real experience on it (configuration, issues resolution..).
I can also give support to the configuration of the system (Apache, LDAP or Active Directory integration, backup).