[General P.]

I installed a 75 gal. fish tank and additional floor joist (2x12) but the floor still sagged a little with the 800 pound load. Would never put that load on upper floor again. If down the basement, thieves would have a more difficult time getting the safe up the stairs than down from upper floor, or right out the door in your case. If you have any portion of outside grade elevation at outside wall below the basement level, just have drain added to the outside; you might have this already and not know it.

And place the safe on a platform in the basement to protect from rust causing dampness from the concrete.

Flooding from the outside elements should not be an issue unless you are below flood grade or have a local landscape problem that needs fixing. If from a broken water pipe, this can be a problem but can be solved by the drain to the outside, or if not possible, a water level control (float switch) that actuates an electric solenoid valve on the main supply in your basement. This type of float switch (and function) is common to air conditioners with the evaporator unit in the basement and is quite cheap.

Oops, looks like I am too late in comments.