I hear your pain. I once had to spend quite a while learning Nagios just to do some simple things. However, Nagios does have the capability do handle the very simple case you are describing. You have hosts [1] and you have services [2] that run on hosts. The relationships are many to many: some hosts run some services, while others run others, or none at all. When telling Nagios about all this, you simply define how to find a host, how to find a service, and finally which hosts to check for which service. This is logical and straightforward in my mind.

Now, the problem is that to get to the links I cited, I had to click through 5 links, and I knew exactly what I was searching for. Nagios's biggest problem is that their documentation looks archaic. Updating that would give the project so much more appeal.

[1] http://nagios.sourceforge.net/docs/nagioscore/4/en/hostcheck...

[2] http://nagios.sourceforge.net/docs/nagioscore/4/en/servicech...