A good 12 ga pump shotgun, with #2 or # 4 shot shells (50 count, can be good hunting weapon as well as personal defense) and at least a dozen slug shells, is first. Additionally, one should have at least one revolver, six-shot, .38 or higher caliber and no less than 50 rounds of ammo (300 is more like it though); alternately, a good condition 1911A .45 semi-auto and hydra shok ammo is good ... nothing like a flying saucer winging toward a perp! It never hurts to have on .22 rifle (in expensive and good for small people and small game) around. Learning how to shoot, clean and store these weapons is a must FOR EVERYONE in the household! Guns can be used to kill, but that should be YOUR discretion, not a perps.
For one trip to the range maybe.
Ammo is like everything else. If you buy a box of 50 for $14.95 at the range, you can probably get 500 for $80 mail order off of the internet. Buy in bulk. It lasts better than TP.
/john