I am administering a server with Ubuntu Server which is running pureFTP.
So far all is well, but I would like to know what I should be monitoring so that I can spot any potential stability and security issues. I'm not looking for sophisticated software, more an idea of what logs and process statistics are most useful for checking on the health of the system.

I'm thinking that I can look at various parameters output from the "ps" command and compare to see if I have things like memory leaks. But I would like to know what experienced admins do.

Also, how do I do a disk check so that when I reboot, I don't get a message saying something like "disk not checked for x days, forcing check" which delays the reboot? I assume there is command that I can run as a cron job late at night. How often should it be run?

What things should I be looking at to spot intrusion attempts? The only shell access is SSH on a non-standard port through UFW firewall, and I regularly do a grep on auth.log for "Fail" or "Invalid". Is there anything else I should look at?

I was logging the firewall (UFW) but I have very few open ports (FTP and SSH on a non standard port) so looking at lists of IP's that have been blocked did not seem useful.

Many thanks