I would check out a thing called Zenoss, it has a community edition which is free and you can easily set it up on a Digital Ocean droplet. It communicates either through the private network, or over the public internet using a protocol called SNMP which gives you stats like Disk, CPU and RAM usage as well as keeping an eye on processes. You can also set it up todo transaction testing such as HTTP and Ping tests. A combination of these techniques can be used to monitor multiple droplets and end to end application availability.
The URL is - http://community.zenoss.org
Hope this helps, any further questions let us know.