Welcome to the forum!
Sounds like you are coming from a Windows environment when you say, "I wish to use one offline to be completely safe and free from constant updates." In a windows environment, those two items are genuine concerns. However, in the Mac environment there are simply NO viruses at this time at all, and the malware that does exist is pretty easily blocked with the tools Sue pointed to in Post #5. I, for example, have no antivirus software, run Onyx about once every other month or so, malwarebytes maybe once or twice a year. I do use Adblock and Ghostery to block annoying popups and some malware. I've been in the Mac world now 9 years, never had an issue.
Updates I have set to prompt me when one is there. I do NOT have autoupdate turned on and never will. But on my wife's system I do have it on, as she never remember to update her software, so it's just better to let it self-tend. But it's YOUR choice.
As for working off-line, you can do that, too. And when you decide to go online there shouldn't be any issues with updating whatever you want, as long as you don't wait so long to do that update that the vendor has moved on 4-5 versions and it's too large a jump for your system. Particularly with the operating system itself, I would recommend that you at least download the installer for each version once, just to have it in a backup somewhere just in case you do need them. Apple is now removing installers for older versions as the new ones come out, so having those installer files can be critical if your Mac gets to the "vintage" state where it is no longer supported by the most recent version of the OS, but you aren't on an updated version at all.