Just bought model #19115....MagPul Fore end, regular butt stock, iron sights. Will use for home defense & targeting under 25 yards viewing above line-of-sight of iron sights. I'm 75 years old with 20/40 corrected vision which will diminish over years to come. What price range, name brand/models should I consider? I don't understand what MOA means or its' relevance to choice. ANY assistance will be appreciated!