Ben...buy a bigger safe than you think you need. They fill up fast. Take my word on it .
Mine is in my finished basement, anchored to the concrete slab from within. It weighs 750lbs and is a Browning. I paid $1400 and change.
The floor is the weakest part(steel wise) of the safe I am told. With new tools, die grinder wheel, plasma cutter ect. Virtually no safe is impenetrable. It will stop the casual thief from kicking in the door and making off with valuables, but James Bond or a pro will get in if they have the time and right tools. I have an active alarm system in my home with the safe being a zone tied to the alarm system. I also have power and light inside the safe for a dehumidifier.
Most safes come with a removable floor panel to anchor the safe to the floor through. If you wish to anchor the safe to a concrete slab, let me know and if you have access to a hammer drill, I will ship you heavy duty anchors to secure your safe to the floor. The garage is a bad place to store it because of the inconsistent temps and who could see it when the door is open. Unless the garage is climate controlled and safe is concealed like a big freezer or fridge..