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Use an AV program if that makes you feel good, or you just want it.
Do you need it? No.

If you scroll to the bottom of your thread here - you will see similar threads listed, probably with exactly the same information that you will get in this thread. You will see both sides there, which will give you recommendations and opinions.
 
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If you frequent shady web sites or install apps from unknown sources, then consider it. Otherwise you should be fine.
 
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Webroot or ClamXav are good. Webroot runs in the background though and may slow the system down (It does not do so on my W10 laptop with an SSD, it did slow down my aging iMac though, but not a problem for the OP.)

ClamXav is solid but you choose when to run it.

OP, enjoy your new iMac.
 
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"MalwareBytes anti-Malware for Mac" is all you need.
It comes in both a "free" and "paid" (subscription) version.
Free is all you need.

As mentioned above, if you run BootCamp, you might want some kind of protection "on the PC side". But what happens there won't affect the Mac side.
 
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Macs are not immune to malware, but no true viruses exist in the wild that can run on macOS, and there never have been any since it was released over 17 years ago. The only malware in the wild that can affect your Mac is a handful of trojans, which can be easily avoided by being careful about what apps you install (no pirated software). 3rd party antivirus apps are not necessary to keep a Mac malware-free, as long as a user practices safe computing.
 
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