Hi there.
I am one of those people who "needs" everything or I feel like I'm missing out, but if you want the full get up, here's what I recommend...
The complete package would be to get the full size step (comes with 4 risers), the slanted risers, and either 4 extra risers or the "high step". Now, you don't HAVE to have the high step, but here's why I recommend it, you will probably need some extra risers to make your step higher when using it as a bench, and for step ups during leg work. So...
You could buy just the extra risers at $11 each ($44) or buy the high step for $59 which gives you the 4 risers and the "high step" top. And I just love the high step for leg only work, for some reason I like it better than the full step for most leg work AND the best part is, because you stack the risers individually, you can make it higher than the regular step without buying anymore risers.
shopcathe.com carries the step and extra risers. Walmart also has the step for $5 less, they also carry the high step, but not the slanted risers. I have never seen extra risers anywhere but on shopcathe.com, actually that's how I discovered Cathe in the first place!
I also have an adjustable bench, so I don't have the slanted risers.
HTH