Low Max is one of my favorites. I definitely recommend purchasing it, even though you've already got the Low Impact Series. There are a few (but not many) elements in the Low Impact Series that remind me of Low Max but nothing formatted the same, with 7 alternating step intervals followed by blasts involving the step. It is true that there are no weight/strength training segments in Low Max. As you say, it provides good lower body conditioning. Some people on this forum are still asking for a Low Max 2 even after the release of the Low Impact Series. As Betty suggested, be sure to watch the video clip on the Cathe web site (I don't have Hardcore Extreme, so I don't know how much of Low Max you've seen).
I just did Low Impact Challenge for the 9th time, making only minor mistakes. Now that I've got it down, I am finding it Intermediate/Advanced (rather than Advanced) and that I definitely need to do the blast at the end and if I have time, the bonus circuit on the same disk to get a good cardio workout. Low Max provides at least as much of a cardio challenge as LIC but it's not a lot more challenging (i.e., it's very doable). Some reviewers of Low Max have noted that the cardio challenge in Low Max is not as great as that in more Advanced workouts, such as the IMAX workouts and Intensity (I agree).
I don't think you'd regret getting Low Max if you want an Intermediate/Advanced low-impact cardio workout with lower body conditioning.