Metabolic workouts are the best! You get cardio and build muscle at the same time. I've never heard about cardio interfering with muscle development? Yes, if you only do cardio but if you weight train and do cardio it should only be beneficial!
ICE Total Body Metabolic is fantastic (up the weights if it's too easy!), Super Cuts, Flex Train... all are a combination of both cardio and strength. The great thing about metabolic training is that you burn more calories than you do with cardio alone as it keeps the calorie burn elevated for a lot longer.
I personally can't use the step (knees!) but since most of Cathe's advanced DVDs include both cardio and weight training you can't go wrong. I'm 46 (eek!) and due to Cathe's varied videos I'm at 17% body fat, 39% muscle, etc. I would say try to do strength for the upper and lower body twice or three times a week and do cardio on the other days (or combine the two).
Good luck!