I've been stepping with Cathe since around 1992, and I learned as I went.
I'd been doing some other step for several months before that, so I wasn't completely new to step, and I'd learned about mirroring the instructor, and some of the basic steps from that.
That being said, from 4DS step onward, I've had to put more effort into learning some of the moves (a combo of more complex moves and cuing not being what it used to be). Some of the workouts are not keepers for me, because I don't have the patience or time or interest to keep working at step moves that take me more than some previews and a couple of times to "get". If I can't get about 85% of a step workout the first time through, I usually get rid of it or never do it again (4DS HIS is one of the latter), especially if there are other factors that make me not like it.
I personally wouldn't want to start Cathe with STS Cardio step, as it would be harder to get, especially if you haven't done her previous workouts.
The "Classics" and "Cardio Hits" workouts pretty much introduce most of Cathe's basic moves, and would be a good place to start with her step workouts (Cardio Hits is on sale right now, but the shipping is high. It's possible to pick it up elsewhere for less).
The outfits are a bit dated, but the workouts are solid: mostly longer, steady-state workouts, but some with little intervals and/or some step-based leg toning. They are more athletic in nature, IMO, with simpler, but not boring, choreography. Though depending on my mood, some of the segments seem to go on a bit long.