Just my opinion...Basic Step/Body Fusion is good, but I like Low Impact Step better. I think the choreography is more fun, and I love how the intensity sections get your heart rate up there. I've read that it will prepare you for Low Max, but I'm too chicken to verify that.
I also recommend Step Heat and Step Jam. The Classics are, in my opinion, an awesome deal for the price. I've been known to mix-and-match parts of each workout.
I have the Kathy Smith step DVD, but I don't think it comes close to Cathe's choreography. But it's still a great workout! I always start out thinking it's so easy, but midway through the 3rd section my heart is soaring.
Best of luck and welcome to Cathe!