You are going to be trapped in a room for 12 hours with a mid 2000s office desktop with no internet connection and an external hard drive; what are you putting on the hard drive?
I've got some DOOM WADs I have been meaning to play so I would probably grab Trench Foot, Total Chaos, and the sequel to Ashes 2063, Ashes: Afterglow with a portable install of GZDoom to play them....