Hey,
Question 1:
Icinga does not have a maximum number of nodes. My Icinga environment has 1x database server, 1x master 01 and 1x master02 and a satellite in each network.
Question 2:
Icinga does not have a paid version. It is Open Source. Icinga is only payable if you have your environment managed by a service provider such as Netways.
Question 3:
You can automate your IT assets. As a module I recommend the icinga director. You can extend your Icinga monitoring functions with your own checks or checks from the community.
For automation, I currently rely on Powershell and Ansible.
Question 4:
Why not? I monitor my ESX and VCenter as well. I do the VM monitoring agent based.
Question 5:
Yes, I also monitor our APC UPS myself. With SNMP you can monitor a lot of things like NAS, Storage, Switches etc… There are many checks on the Internet, some I have programmed myself.
My recommendation is that you read up on Icinga in the Icinga documentation. The advantage of Icinga is clearly that it is open source and therefore has a lot of freedom. I myself check over 2000 services and monitor IT and non-IT solutions. Non-IT in this case is monitoring pumps or temperature of server rooms. Learning Icinga » Documentation, Live Demo, Trainings