Are builtin security features allied to secure user behavior enough to protect a Linux system? Or should I install an antivirus software?
If its a company server then yes you should have anti-virus on it. If you don't and it gets infected, you will either get your balls kicked, or get fired. Neither is good.
At home, you can take more of a chance. however, if you have windows machines, the Linux samba shares can act as safe haven for the virus and can keep reinfecting your windows machines.
Whilst it's quite rare to catch a linux virus or worm, but they do appear from time to time. Having a SSH port available to the internet is a major well known risk, there may well be other common attack methods for linux home servers.