Used guns are always possible, but just make sure that you really look over what you're purchasing. With your budget, you should have no problem buying a brand new gun and that will include a warranty and piece of mind that the gun is in perfect condition.
The Knight Little Horn and Big Horns are great guns for sure, but I would also consider either the CVA Wolf or Optima, depending on personal preference. The Wolf has a compact version, with a 13" length of pull, but your wife will be able to handle the full size version with the 14" LOP since she is taller. The Wolf also has a shorter barrel, and is lighter weight, which may be better for her as well. Out to 100yds, the Wolf will be perfect. If weight isn't an issue, I would go with the Optima. You'll gain some better accuracy at the longer yardages, even though with open sights you'll be limited.
It will boil down to personal preference and what you like, but any of these guns will be great. Let us know what you choose, and how she likes it!