I just bought a nice safe.
In addition to the 'bigger than you think' advice, I suggest the following.
1. Don't bother with an 'interior' unless you are completely helpless. I ordered an 'interior' that looked good on paper. They gave me exactly what I ordered. It didn't work. Shelves were way too big, the long gun racks were too crowded and didn't have space for the butts and so forth. I ended up making my own interior, making far better use of the space.
If you can't work a circular saw and such, then get whatever appeals to you. I wasn't happy with what the factory visualized.
2. I ordered the 'carpeted interior'. I should have left it out and installed my own carpet-heavy cloth-panelling-whatever to go with my own interior racks. By the way, regular carpet is really difficult to install in a safe. The safe companies use a lighter material; I'm not really sure what it is.
3. Do something about humidity. I ordered a de-humidifier with the safe. I think it's working, but it isn't obvious. I'm not sure if I would get the factory de-humidifier or get a 'Goldenrod' and install it myself. Have the safe company drill the hole for the wire for you.
4. Lights. Even in a well-lit workshop, the interior of a safe is pretty dark. I think I'm going to get a string of white 'christmas' lights and install it in the corners and edges of the safe. The safe company I bought from will install them, but with changing the interior, I would have probably regretted getting them put in. It still needs interior lights.
5. Location: A concrete deck is best. Figure on having some small scraps of wood to wedge under corners to level the safe and keep it from rocking. Also plan where the door will open. The door is heavy; it is hinged and balanced well, so it's not hard to open and close, but you don't want to get your hand or the cat between the door and the wall. The door has mass and therefore momentum. Also, if the door runs into the furniture or wall, expect some marks. I have a wooden 'bumper' to catch the safe door.
6. Get the change key and learn how to 'set' combinations. It's easier than loading match ammo; far easier than shooting bullseye well. You want to be the one who knows the combination to your safe.
7. Safes are not designed to be opened from the inside. Keep that in mind.