Don't know about the sump itself but you don't want lava rock in it. Lava rock is primarily made of silicates which will constantly leach into the water. You will have massive, endless diatom blooms. In addition the rock could contain any number of toxic heavy metals that will react with seawater.
You also won't want the bioballs. Although effective at removing ammonia and nitrite from fish only systems, they do nothing to remove the resulting nitrates. High nitrates can be harmful to fish and even worse for corals and other invertebrates.