Glad you found us at 3FC. I'm 51 too, so I can assure you that you CAN lose those extra lbs. even past 50
I calorie count, so I can't help you with your South Beach question. But, I will offer one small piece of unsolicited advice - pick a plan that you can do forever. I "dieted" my way to 215 lbs over the years. It was only after I learned to change my lifestyle and incorporate healthy portions into my day, that I was able to finally shed those lbs. AND stay at a healthy weight.
Don't think about being "good" or "bad" with food - just find a plan that works for you and forge ahead.
I also found that journaling my eating really helped me stay on track. I like Fitday, and others around here like theDailyPlate. Both are free and are GREAT resources.
Of course you have found one of the very best resources of all in 3FC. The folk here are amazing!