I personally don't think that a single shot anything is a good HD gun... you may want to look into a used Remington 870, Winchester 1300, or Mossberg 500. You could find a double barrel gun, too, but those will be more expensive (usually) than the pumps and will have lower ammo capacity. You may also want to consider a semi-auto shotgun (sometimes called a "gas gun"). Any pump (or especially semiauto) shotgun in 20 gauge should be low recoiling and you can get decent buckshot loads in 20 gauge. The single shot 20s are nice, but not for HD purposes... they also tend to kick quite a bit more than their pump counterparts. There are also used .38 spl revolvers to look at, and bulgie maks... how limited is your budget, exactly? I think a lot of people have found used Smith & Wesson Model 10 .38 spl revolvers for ~$110, which is about the same price as the single shot shotguns from NEF.
(sorry if my post is a hard read... im in a hurry, but i should be back later to clarify this post a bit better.)