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Nagios has a plugin to monitor mail queue of a cpanel server. You will need to install and configure it.
All sounds good. You already have a plan in place which looks nice.
For technical issues - I assume that you have a good knowledge of the industry and can handle level 1 and 2 support for your customers. If you...
You can use nagios or zabbix to monitor the server.
And Yes, I also heard about Pinguzo. Have you used it?
I would recommend to have a niche product and then start contributing on their communities. Like for example: "VPS Wordpress Hosting" , give...