This post is showing you if you have Windows Server 2016 then you do not need to buy a separate Antivirus software
Obviously in Windows Server 2016 you can see it is updating Antivirus during the Windows updating process. So you might want to stay with it for a while before spending extra cost for other Antivirus software. As till now, there is no hard evident to proof other Antivirus software is catching virus better than the Windows Defender.
You can find the Antivirus Windows Defender showing below.