Congratulations to you and your wife for wanting to lose weight and be healthy! I'm glad that you are realizing it is a lifestyle change, not just a diet. That is a great start and a wonderful mindset. I started on the same journey 9 years ago and managed to lose 35 pounds and keep it off.
I'm not very familiar with South Beach. For me, I started reading food labels. I didn't follow a particular plan, I just changed my eating habits on a very gradual basis. First, go through your cupboards and toss out the bad stuff. Don't buy it and don't bring it home. I started eating more chicken, fish, veggies and fruit. I'm not a big veggie eater but I ate what I liked. I cut all simple carbs (anything made with white flour) and started eating whole grains instead. Allow yourself 1 cheat meal a week so you won't be deprived. I found ways to satisfy my sweet tooth without going overboard. If you are doing South Beach, read all you can about the program. There are so many wonderful resources on the web. Pick up a couple of fitness magazine that will also provide you with information. I read whatever I can get my hands on regarding diet, nutrition and fitness.
As for exercise, I'm not sure what your fitness levels are. Cathe is advanced so if you haven't worked out in a while, you might want to try walking or checking out other dvds. Cathe has a wonderful selection that you can review in length on this website. I started out doing Taebo and mixed in some walking. I'm not big on stepping so I can't recommend any of her dvds but I do love her circuit workouts (Boot Camp and High Step Challenge), Kick, Punch and Crunch and any of her weight workouts.
There are many knowledgeable people that visit this forum. If you have any questions, please feel free to ask.
HTH,