Hi, Deborah, and welcome to the gang! Never underestimate a Cathe fan's talent for spending your money for you!
In all honesty, I think it would be just a bit of a leap to go from the step routines you currently have to Rhythmic Step, which is probably the most complicated choreography (and the fastest-paced) of her step routines. Rhythmic Step could be a goal to work toward.
For cardio, I'd suggest the following:
Cardio + Weights (on the same DVD as I-Max 2); the intensity of the step routine is a good step up (pardon the atrocious pun) from Basic Step and Low Max. (If you're already doing I-max 2 comfortably maybe you're more fit than you think, and can consider the other workouts on the Rhythmic Step DVD, Maximum Intensity Cardio and Interval Max). Also consider the Cardio Hits 3-workout DVD, including Power Max, Step Fit and Step Works; these are listed as Intermediate in the workout precises. Also consider Low Impact Step and Body Sculpt DVD, as well as High Step Circuit; these are intermediate cardio and strength routines.
For strength, you've got a good one in Muscle Max. Power Hour is more of an intermediate, endurance-oriented workout along the lines of Body Pump. It comes on the same DVD as Body Max (step, step-circuit cardio and upper body and ab conditioning) and Maximum Intensity Strength. That DVD would be a good investment for all three workouts, as you evolve from endurance to strength training.
For true strength training, certainly consider one of Cathe's split-set systems, such as Pure Strength (three workouts hitting select muscle groups), Pyramid Upper / Lower Body (two workouts hitting . . . well, upper and lower body) or Gym Styles, another 3-workout split-set series. A goal series to work toward might be the Slow and Heavy series, another 3-workout split-set series that is very focused and designed for premium strength and mass gains.
HTH -
A-Jock