I do not believe there is any mac installations that are easy to do.

"There are no official releases of Armory for Mac/OSX, because of the difficulty of packaging applications for it.  If you are not afraid of the command line (terminal), then you can follow the instructions on the Building Armory from Source page."
https://bitcoinarmory.com/get-armory/

Are you familar with Ubuntu? If the computer will stay offline, why bother? The risk of downloading a virus isn't there, unless it comes in the way of your usb stick. I'd stick with something you are familar with, and at most fresh install the OS with a PAID version if windows, not downloaded torrent.