As far as "vault doors" browning sells ao couple, but they are the same price or higher than their mid priced gun safes (1900+). For me, the new house I just bought (closed escrow yesterday) is only one story and does not have a basement, but it does have an 8x10 tool shed out back. I am going to reinforce the walls, ceiling and put in 45 minute steel doors (double set of doors). I was thinking of using the chain link fence layered between the gypsum board for the ceiling (couple layers) and then the 1/8 metal sheeting and then a 1 or 1.5" plywood, then the chain link, then another layer of 1/8 sheeting and then a final layer of 1.5" plywood. all the seems would be channel or angle iron as well as the door frames.
After reading about the fire suppression system I think that is a great idea. I would probably go with the powder type of system. Also was thinking of rigger and external alarm (loud speaker and strobing light) along with motion sensor lights and of course sensors on both doors and a motion sensor in the room if they manage to bypass the door sensors. The alarm and fire detection/suppression would be hooked to the existing 110 power, but would each have a seperate UPS (backup power) in case of the burglar cutting power to the shed thinking that would kill the alarm.
As far as turning one of the walk in closets in to a gun room, my wife would kill me, so I wouldn't even try. More power to ya for getting the wife to go with this idea.