I cant really add anything useful, but i must say, "only one gun" it doesn't work that way!
This was me earlier in the year:
First i was looking at a 3gun safe, 1 x .22 1 x air rifle and a milsurp says me....but then i found too many milsurps i simply must have!
5 was never an option after this.
I then filled (in my mind) a 10 gun safe pretty quickly!
So i ended up with a 14 gun unit
Don't go cheap new safe, the units with folded sheetmetal doors with no filling or inner skin can be pried very easily, a mate has one (for ammo) and he showed me how easy these are to defeat and INSISTED i not buy crap.The unit i purchased used weighs well in excess of 200kg,fire proof blah blah and cost about the same as a dirt cheap brand new 10 gun safe and the door alone weighs more than one of these cheap 10 gun safes. Ebay/gumtree is your friend! I got a separate decent sized fireproof safe for ammo,mint condition, for $10 of gumtree.
Buy once cry once they say and i agree.