but there is one thing you should be be aware of and that you will have to update the OS to at least OS 10.11 as that is the lowest OS version that the newer versions of MC will run on.
And I wouldn't be surprised if the system requirement increases with MC32, e.g. minimum becoming 10.12 or 10.13 or anything newer.
How about running 10.11 and that compatible MC version?
The real “rub” is no internet connection.
I can move the iMac to an internet connection for the installs.
Will the OS need an Apple account? Does it need to see the internet daily?
MC31 runs on 10.11, but keep in mind MC needs an internet connection when adding your license, then it should work without a internet connection. If it somehow loses the license (e.g. changing hostname, using MAC address randomization, wrong time used, etc.) then it'd need to be reconnected to re-add the license. If system requirements increase for MC32 you likely can't use it without updating your OS too. Depending on the make and model of the Mac, it may be possible to force upgrade to a newer macOS version like Sierra, High Sierra, Mojave or Catalina, depending, it has a little bit of risk though. More info on that here (I'd highly recommend reading all of them and looking at known issues and whatnot if thinking about going this route):
https://dosdude1.com/software.html