Real lava rock, or the stuff sold at many LFS called lava rock? Entirely different rocks--the stuff sold at the LFS is safe, I've used it for many years.
If you're doing a fishless cycle, you can hold off on the skimmer. If you're cycling with live rock, adding it now will be beneficial to the survivors on the rock.
I have the Coralife Super Skimmer on my 55g and I love the job it is doing. I am able to set it to skim wet or dry depending on what I want done at that time. For the most part I leave it set to skim dry for a darker, thicker collection of junk in the cup. As OrionGirl said, any of the three should work fine for you but even though I have the Super Skimmer if I had the chance I would probably go with the Aqua C skimmer myself.