An organic turkey that hasn't been frozen is your best bet when choosing a bird. You can even find wild turkeys online. If you're pinching pennies, a regular frozen turkey from the grocery store will do if you have the right tricks up your sleeve.
You will need about 3/4 of a pound to 1 pound of turkey for each guest you will be having for dinner, factoring in on how much you want to have left over. If you have a large group that requires more than 16 pounds of turkey, it might be best to buy two smaller turkeys, since larger turkeys tend to be tougher and lose their juices more easily during cooking.