If your computer is totally cut off from any external input,there will be no security risk at play IMHO..But Windows PC have those update to be installed because it will improve your PC integrity and also fix some flaws that maybe bugs the OS itself..For example it might solve most blue screen problems or force closes..Then again if you're not even exposed to external input,those updates doesn't really matter.

Using a free antivirus is enough for casual use and if you're not downloading collections of movies,dramas,games and what not..Windows Defender should be enough too..

So to answer your question..No and no..I might be wrong though..Another thing,Linux PC do have malware threats too you know?And it's a lot dangerous than average viruses..