Your translation isn't too bad. Here is my suggestion: "Good mountains and good water make great teas, with the essence of Guanyin and the strength of iron". Alternatively: "Great mountains and great water yield great teas possessing the essence of Guanyin and the strength of iron." You could potentially include synonyms for "good" or great to make your text sound more literary/poetic in English, although some may find that distasteful, since the original text uses “好" for the adjectives.