I've been an aqua group fitness instructor for over 7 years now. Aqua can be a superior cardio AND muscle endurance workout combined when the instructor and participants know what they're doing. The "aqua weights" you are referring to are hand buoys, and can be used as an ersatz float device or as a piece of resistance equipment, both during a cardio segment and during a stand-alone upper body resistance workout.
I would try a class on its own, not having performed a step class prior to it; if the class being offered is an intermediate or high level intensity class than IMHO doing a step class beforehand would be too much. After gauging your exertion level with the "debut" class, then decide if doing step beforehand is too much.
The intensity level of an aqua class is much the same as the intensity level of any studio / land class: that depends on the instructor and how the class is described.
A-Jock