As a beginner I hope my experiences purchasing a safe (or more accurately RSC?) will help others with their choices. Some of my earlier thoughts are described above.
Based on my research, the excellent posts in this thread and elsewhere in the forum, and discussion with forum member a1abdj, I bought a Diamond Back GS5928H from a1abdj, who was very helpful and informative. This met my requirements, which are to safely and relatively economically (although this is still $$$) store firearms away from very inquisitive children and their friends (especially when they are all older teenagers and the parents might not be home) and casual and even not so casual burglars. The safe has a 1 hour fire liner, but I purchased it mainly for safety.
I chose the Diamond Back H series because it has a LaGard UL rated mechanical lock compared to the L series lighter duty non-mechanical lock. In addition it was stronger compared to some other safes I was looking at for only slightly more money. I could have spent even more for an AMSEC or similar, but had to stop the spending somewhere! I specifically did not purchase an electronic lock, not because of its increased price, but because I thought a mechanical lock should provide better long term reliability over 10 or 20 years. I bought the GS5928H instead of the GS5924H because I felt a wider door would provide better light and access (and it does).
I had the safe delivered to a local SAVTA member who moved it into my house and down some stairs. Although they did not require the stairs to be reinforced for this safe (540 pounds) I felt better reinforcing the stairs as described by CB900F
here. With some additional securing this had the significant additional advantage of making the stairs completely solid and stopped the squeaks, creaks, and groans that had been bugging me for a long time!
I mounted the safe on a 3/4" plywood base over bare concrete, with the plywood base 1" larger in width and depth than the safe providing a small overhang. This, plus spacing the safe slightly off the back wall, keeps any damp from direct contact with the safe. I then leveled the safe (so the door does not swing by itself) and secured it through the plywood to the concrete with the supplied concrete bolts. Securing the safe is a must to prevent it tipping forwards when you open the door! In addition, it provides additional security in preventing easy removal of the safe.