A safe is a very good investment. It’s a durable good with few moving parts. It won’t go out of style or fashion. Everyone has a use for a safe. Once you have a safe, you have a safe for life. Actually your safe will probably be around after you are gone. Other than your home, I can’t think of any other item that you are likely to will to your children besides a safe, guns or precious metals that will retain their value and require virtually zero maintenance.
Since we are talking the long term I would highly suggest a dehumidifier. There are rod style electric heaters and rechargeable pellet styles heaters available. Most are good and affordable. This will keep your guns and valuables from rusting out.
Besides guns you will want to store your irreplaceable heirlooms in your safe as well. This will insure that your grandchildren and great grandchildren will have a physical piece of their family history that is protected from fire. What a great way to be remembered, a big, heavy, secure safe.
So be sure to not size your safe for what you have now. Size it to hold everything that you may want to give to them as an inheritance. A conservative, neutral finish like matte black may be a good idea too.