I found that for me, the best way to get to the point where I could do Cathe's toughest workouts was to just start doing them. For example, I probably wasn't quite advanced enough for IMAX 2 when I bought it, but I just started doing it, say five intervals at a time, all the while modifying anything that was too difficult. You can always take the power out of some of those jumpier moves (do regular jumping jacks instead of plies, don't jump all the power 15's, etc).
You might want to try Cardio & Weights, which is a circuit workout, but the cardio is not overly intense. Also, the CTX Series is a great one to learn Cathe's moves from. It's got a lot of high impact moves, but the cardio sections are short, so it's fairly doable for an itermediate exerciser.
So, if I were you, I'd probably go ahead and get Cardio Hits, and just take your time working up to it's intensity.
Shelbygirl