With the way things are going these days, it's worth a look on the usual suspect internet sites to see what's available second hand to save money. Brattonsound, Infac Safe and others are good makes, but there's others. The ones that need just the one key to open are a lot handier. Or you could go the way of a combination one. They tend to cost more. Also perhaps worth looking at a separate safe for your ammo if space allows. Stand alone safes used for ammo aren't expensive because they sell more for keeping jewellery, money and the like in. Any hardware store will have those in various sizes, or the usual internet outlets.
But the best advice is to get more space than you need. Given time, the amount of guns/rifles you end up with may increase, you might, even if you aren't intending to buy more guns, buy one and later sell one, so spare cabinet space is always handy. And there's always someone whose giving up that wants to give you a firearm or sell for a pittance that you can't turn down.