Nagios is in my opinion not for anyone. Dependent on your skill level, the skill level of your staff, and/or the commitment to IT from management there might be better solutions.
Solarwinds NPM is an expensive tool that you buy, that works for some people. In my experience it has a tendency to scan and rescan your network and add all these services that you do not want to monitor.
I stick to nagios, I have in the last 10 years not been stumped by coming across anything that nagios cannot monitor.