initially, you could get away with:
some rock
a cheap-ish skimmer
a couple of grunty powerheads for current
any maybe use a canister for carbon - dont use them as you would on a freshwater tank, with all the filterwool bioballs etc.
if i was starting again, i would avoid sand for the first few months, and have heaps more current in the tank, along with as good a skimmer as i could afford - try to get one second hand.