Some suggestions. First go visit the sigforum.com (the same owner who used to own this one.) They have a resident safe salesman. He participates in other discussions, but he's good with safes.
Then read the older discussions here and there on the subject. This isn't a small purchase, and you should do your homework first.
Yes, putting it out in the garage opens all kinds of issues. Good and bad. Once its where you're putting it, there is no real reason to buy new. The easy access you're concerned with can also be used to your advantage. Look for a much better industrial quality safe, but used. Its easy to install if its in the garage. It does take a rigger to deliver, but its much easier to install than a gun safe in a basement or upstairs.
Gun safes aren't real safes. They're just sheet metal cabinets with fancy looking locks. They don't keep out anyone with metal working skills. Even a guy who's good with a sawsall and some time will enter your safe. But a real honest to gawd safe will have inches of concrete as well as layers of steel. And the billybob pickup truck will pull its bumpers off trying to move it.
And my 2nd guess will be for you to frame up a corner and pour 8" of concrete into the shape desired. Use some steel for added strength, then put the best quality door you can buy. All the safe companies make them. Oh, when I wanted a 10x10 safe, a friend said "why so small?"
I would have to respectfully disagree with some of what you said.
Most well made gun safes are plenty secure for the average home owner.
A sawsall? I don't think so. No thief has that kind of time nor the desire to remain in the house long enough to use one. Thieves attack what they think are the weak points of the safe. Hinges, dial, and handle. They attack the parts of the safe that actually make it harder for them to be successful.
I've seen safes left out in the desert where dummies tried to get in but couldn't. Generally, thieves don't have the know how, the tools, or the time to mess with an average joe's safe. Someone with those kinds of skills is either honestly employed like myself or busy trying to rip someone off that they know has lot's of money, gold, or diamonds at home in thier safe.
To the OP, I've been selling, repairing, and servicing safes for almost 20 years.
Definately buy one size bigger. You WILL find more to put in it.
Electronic locks are nice and convenient, but more prone to fail.
Have it delivered and installed. Preferably during the middle of the week when your neighbors are at work.
I highly recommend Liberty, Cannon, Amsec, and Fort Knox. There may be others that I can't think of now.
Stay FAR, FAR away from Sentry or anything else made in China. You get what you pay for. I've opened more Sentry safes than I can remember because they broke internally and cannot be repaired and parts are not available.
Think of it as an investment.