Simply put, there is something very wrong with your boss's iMac that he or she ought to fix. I have a number of 2011 model year iMacs that run El Capitan perfectly well, faster than any previous OS X version, quantifiably so, according to popular benchmarking software. Therefore it stands to reason whatever is wrong with your boss's iMac can be fixed.
The GUI is a matter of personal taste. If you prefer to use Mountain Lion that's fine, as long as you are aware that it is no longer supported and will no longer receive security updates. That answers your stated concern: I just want to be sure using it as long as i can without security issues in the near future.
Even if i navigate in a suspicious website can i still be attacked?
Merely navigating to and loading a suspicious website cannot result in any harm on a Mac, even if the website appears to "lock up" your browser. If that should occur read Phony "tech support" / "ransomware" popups and web pages for the solution.
I have Hands Off, AdBlocker and Firewall activated.
I have never used any of those things. They can be useful, but are not necessary, can cause unexpected behavior, and will often result only in your own inconvenience. The OS X application firewall's purpose is frequently misunderstood, and is completely unrelated to protecting your Mac from malicious software. It has useful purposes though, most of which have to do with using a portable Mac on a public Wi-Fi network shared by people you don't know or trust.
Following the applicable suggestions in the User Tip I posted will preclude falling victim to popular scams that seek to defraud you, regardless of the operating system you're using. The most important concept to keep in mind is that no software, including OS X itself, can protect you from all threats. OS X's built-in security protects itself from modifications that you do not explicitly authorize, but overall device security is your responsibility. Part of that is keeping its operating system up to date, regardless of the uninformed opinions of others.