Good sharks for that tank size are bamboos/epaulettes, coral cats, marbled cats, and maybe horns, although they might get too big. What are the dimensions of the tank? With a shark you'll be limited to maybe 2 other fish because of the tank size. Eels, groupers, and lions do well with the above species. Stay away from puffers and triggers.
In my 180g(with a horn and whitespotted bamboo) I have about 40lbs of lr I think. I wouldnt recommend you go much over that. Just enough to make some caves and stuff for the shark to hide if it wants. Make sure the structures are secure though. The sand I use is some aragonite I bought from a reefer who was selling his tank along with two bags of ls. Home depot sells stuff called southdown(also called tropical playsand) which is good.