7 or 8x is usually a good bet, although in open (desert) country 10x might be desired. Porro prism usually get you better optical quality than roof prism at the same price point. 30 or 40mm objectives are the standard sizes. Higher magnification will benefit from larger objectives. Very important to check the eye relief of each model considered, especially if she wears glasses. Hard to go wrong with Leupold or Nikon in that price range. Stay away from made in China models.