Monit and Pushover - $5 and I was done [1]. Since implementing this I have had only one problem [2]. The great thing is the servers just fix themselves and I don't have to do anything - this is really nice since I am the person on call.

1. https://www.tillett.info/2016/07/19/dead-simple-centos-serve...

2. There really wasn't any problem I had just set the resource utilisation value too low so I got a false positive.