Remington 1100 or 11-87 would be the most cost efficient, especially if you are looking in the used market. Although if you found a Browning, Auto 5 for a reasonable cost they are a great shotgun. Any of the modern guns are safe, just find what you like and fits you.
Unless you plan to load your own ammo, or fall into a smoking deal, the 12 gauge is the most cost-effective ammo to buy, followed by 20 gauge.
Doug