I agree with Terry. Get the BIGGEST safe you can. It's a lifetime investment, so the money will be well spent. I would also suggest that you get the HEAVIEST vault you can. This will be a good indication of how much metal will be between your guns and some guy wanting to get to them while you are away from home. If he has two buddies and a pickup truck with him, you DON'T want them able to just pick up the vault and haul it away with them, now do you?
How much does a top of the line vault cost? I got a Fort Knox vault a few years ago and it is one huge box. I think it cost me around $2,500 plus the guy brought it here and put it in place for me . At the time, I had over $8K of guns laying around in the building that I could not fit into my first vault. So I think it was a worthwhile investment. The Fort Knox vaults are excellent pieces of workmanship, in my opinion.
It does give me a LOT of piece of mind. When I was younger, I used to have a very nice wooden gun cabinet with glass doors in my bedroom. Looked beautiful with my guns neatly arranged in it. But everytime I would come home from being out, I would make the bedroom my first stop to make sure my guns were still there.
Oh, another suggestion. Get the electronic push button lock for your safe. I don't know why, but I just like it a LOT better than the manual combination lock. Plus my wife can figure out how to use it. (Of course, to some people that would be a drawback, I guess.)