Thanks to Kevin, a friend from France, I recently discovered "hetrixtools" (link attached) which lets you monitor one or more hosts for free (there are limitations but they are not strict): without installing any software on your machines, you can do #PING, #Website, #SMTP and #Heartbeat checks over your hosts; then if you install their agent (it's really straightforward but #CPU usage was an issue for my single-core VPS) you can get a lot more, from services to ports monitoring, to disk health checks.
So, after I saw that installing the agent was not feasible for me, I decided to try a local run monitoring service and I choose #Monit, which is well documented and all in all quite easy to set up once you get the way it works.
I've got #notifications directly to my #INBOX, via my own #email server and here you can see what I'm monitoring so far.. https://i.imgur.com/uqDceAa.jpeg
Each of those items is click-able and gives a lot of detailed info. I cannot see any difference in #CPU usage on my host and am quite satisfied by the results.