The most common thing I see happen with safes, is people tend to buy small and cheap. Then as a collection grows, or they discover all the things they can keep in there, they want to upgrade and end up spending a lot more money.
Decide first, all the things you may want to keep in one. Guns, knives, jewelry, coins, valuable papers, and other collectables.
Then, I looked at fire resistant. How long will one protect paper in a typical house fire. Paper goes first.
Then, a quality safe is forever. Will be passed down to your children and their children.
So, we bought the largest one we could find. Within the safe, we have a couple of the smaller sentry fireproof lock boxes that contain valuable papers and jewelry.
We also closed out the safe Deposit box at the bank and moved that to the safe. Then I calculated that in about 60 years that alone will pay for the safe.
We have had ours now for 7 or 8 years and haven't run out of room yet. But it is a big huge thing and you have to have a place to put it.
your collections might grow, a small safe won't.