Bolting down a safe is a good idea. If someone got it in, they can get it out. I've known of 1000+lbs safe being stolen with pallet jackets, etc. It does happen.
I wouldn't use those heating elements in a safe. You need desiccant gel inside the safe or something similar that absorbs the moisture. You don't want to just keep things warm because the water has nowhere to go inside a closed safe.
Think of your car getting wet inside and you just put it in the sun with all the windows up. All that happens is the moisture goes into the air and covers everything with condensation. What you need is to pull the moisture out of the car and get dry air in there and water out. Getting it hot by itself doesn't solve the problem. Plus, I don't personally think it is a good idea to put any kind of electric heater inside a safe you want to be fireproof. Imagine having the heating element cause the fire inside your safe! Just something to think about.
Buy some of the gel canisters you see at Midway, etc and put them in the safe. Every now and then check out the color and if they change you just put them in the oven for a few hours to re-activate them by cooking out the water. The canisters pull water out of the air that got inside safe when you opened the door. That's what you really want.
You can put the safe directly on concrete. It would take years of exposure to see any kind of rust on the painted underside. And that is assuming you live in a really wet/salty area. Mostly it's a non-issue and the safes are made to just be put directly onto the surface. If you raise it off the ground you expose it to pry attacks.
You may also want to place it out of view from the street when your garage door is open, and/or toss a blanket over it to not make it so obvious what it is from casual observers.
If you have a monitored alarm system, put a smoke/heat alarm above your safe. It would activate if someone used powertools against your safe and dispatch the fire department to your home. Since the smoke alarms are active 24/7 it would sound an alarm even if you forget to set the alarm when you leave.
Also with that said, don't leave angle grinders, etc. in your garage that can be used on your safe. Consider placing such power tools inside your safe so you don't make a crook's life easier by leaving them nearby to give them any ideas.