I'm thinking the basement water is just a "holy crap it just rained bucket loads" problem. I dont think it gets consistently wet down there. I think If I can get it to stay dry I would definitely put it down there, I just have no clue as to what the first step to "sealing" the foundation would be. The safe is already about 4 inches off the ground on huge casters. The thing is huge probably 7ft wide 3 or 4ft deep and 6 feet tall. It's a beast of a safe and is perfect for what I need it to do. It's just way to have to put anywhere upstairs I think.
Step 1 is to identify the type of foundation you have. The easiest to deal with is a pour congrete basement, then stone wall. If it's not one of those two, you'll likely want to consider building a poured concrete foundation under the house.
Step 2 is an exterior foundation drain. Most I've seen run 3-5k. They dig out the backfill around the foundation down to the footings add a few inches of gravel, then install 4" diameter half-perf pipe around the perimeter of the foundation. Once the pipe is in, it's covered with 6-12 inches of gravel and covered with dirt (I've seen a couple where they put 6" of gravel along the foundation wall all the way to the surface - this is prefered for stone foundations to keep water pressure from building outside the rock wall of the foundation)
Step 3 Weather seal the basement wall - This is most effective on the outside before they backfill over the perf pipe, but it needs to be allowed to cure before it can be covered. Typically this is tar or a thick rubbery expoy paint to water proof the outside of the wall. I can also be applied to the interior of the wall (and should be if you have a rock wall basement
Step 4 - the foundation drain needs somewhere down-hill to flow. If your house is not built on a hill with enough slope to discharge above ground below the floor of the foundation, you may have to consider installing a dry-well (a 3-4' diameter hole several feet below the floor of the house and filledwith some large rocks to drain water into) or even a dug well (my house had the later already on property and not used for house water, so we ran the discharge there) You can add a pump to the dug well and use that for lawn irrigation without feeding off the main houses water supply
If you're going to finish the basement, apply a water proofing epoxy paint to the interior side of the foundation wall and then a vapor barrier. Use stainless steel spacers to hold 2x4 (or 2x6) off the ground and lay them along the edge of the walls to be finished and then nail/screw a matching board to the bottom of the rafters directly above it. Space both from the wall with vapor barrier with at least a nail thickness.
Frame 16" on center - You can use 2x3, 2x4 or 2x6 for framing, but you want to keep them flush to the foot and header boards on the interior.
Insulate - since you have the vapor barrier on the wall you can use kraft faced fiberglass batting, but sprayed in polyurathane foam insulation adds even more water proofing (especially if the framing boards are an inch or two off the foundation wall)
Finally, wall it over with bathroom wall board just in case some moisture does get in, it will resit mold and mild intrution.
That should water proof the basement, but if you want to take it a little further:
Some companies like to cut 4" deep 3-4" wide channels in the slab of the foundation, just inside the foundation walls (would be under the 2x6 floor board I described above, so you may need some extra spacer hight if you're doing that) and cover them with a metal grate. The bottoms are sloped to a sump hole and a pump is installed to remove water from the hole and out of the house (and into the foundation drain system if you have one) this removes any water that might get into the basement.
If you have a sump hole, an active pump dehymidifier compresses the air, forcing water out of the air and into a collection tank. This can be discharged into the sump hole or, if the dehymidifer is raised, down a standard drainage pipe after a trap.
For the size safe you're talking about, I would definately recommend something to raise it off the floor. If you have the ceiling height in the basement go with the concrete "H" blocks. Mark out an area on the floor that's about 6" wider than the safe with an inch spacing behind and flush to the front. Lay a row of blocks along the front edge with equal spacing between each block. repeat 1" in from the outside wall
Place 2 H blocks along the sides, again their long dimension following the perimeter of the safe. Then build a line between the center of each of the front blocks and the corrisonding back block for the mid-supports. That should be more than sufficient to support the safe while allowing air to flow under the safe to prevent moisture trapping and rusting
You need to cover them with something. sheet stone (forget the proper name) large enough, cover it with that. If you cant, 3/4" thick plywood (sealed with polyurathane) or non-arsnic pressure-treated 2x6s to build a "deck" to place the safe one. (space the 2x6 at least 1/2" if you're using them).
Run wiring into the safe for lights dehumidifer and heaters and you're in business.