Wow, I did not realize the price drop on Amy's recent DVD's. I have most of her workouts and ASC4 is my favorite of her step workouts - probably because it does not seem as complicated or intricate as some of her other step workouts. ASC5 I would only recommend if you have a lot of patience. I have done it a few times and still trip over myself -- because she throws big blocks of choreography at you with little warning and they are pretty complex. So you really have to do this workout a lot to get it. I think that ASC2 is a bit easier to pick up than 5, so it depends on what you want.
In regards to Body Box, I only tried it once and I would not describe it as typical kickboxing. It has some martial arts elements that slow it down and make it more complicated to learn with some turns and different things going on that are hard to describe. I only tried it once and have not picked it up since, I should give it another shot - it did not seem too intense and it tested my patience.
I think that Kickbox Extreme is probably higher intensity than Body Box but you really have to like Amy's style because she does have a different style than other kickbox workouts I have tried. I would go for Kickbox Extreme instead of Body Box.
If you like hi/lo, I think that she excels in this area. hi/lo dome and hi/lo dynamics are two of my favorites.
Hopefully others will give additional input for you!