Dear Simian, The 870 should make a good security gun with just the addition of a pistol grip to replace the stock. This addition will make it much easier to manuver in the close confines of your home.
You should spend some time at the range practicing with your new gun. Use target loads for this at first as in the beginning you'll want to learn to: look at the target, lead the gun towards the target with your left hand (if you're right handed) as you raise it to eye level, then fire. Space some tin cans out on the ground for targets about five yards in front of you. When you are consistently centering the cans with your pattern you can start practice with buckshot on silouette targets. You'll be proficient before you know it.
Dry firing at home will help a lot too. Especially if you don't have much experience with pump guns. Once you get used to pumping it you'll never forget (like riding a bike). Dont worry about dropping the hammer on an empty chamber it won't do any harm. Enjoy your new gun and stay safe. Gary