Basically...
-TEST KITS VERY=IMPORTANT!!!
-HOB (hang on back) filter or canister filter
or
sump(/refugium) which requires plumbing, an overflow box, and external or internal pump
-lights (of course important on a reef setup, and is usually the largest investment if you do get corals) LED, T5, T5 HO, VHO, Metal Halide, Power Compact
-protein skimmer (optional, and if you keep up on water changes, probably don't really need one for that size tank)
-Sand (optional; can go bare bottom)
-LR (live rock) or dry "base" rock: pros and cons of both. I went with 100% dry rock so that I wouldn't get any pests, but the LR comes with beneficial bacteria,worms,pods, and more that you won't get unless you add them separately.
-can have filter sock, floss, etc, carbon.
-powerheads (if you want additional flow)
-salt
-RO/DI system (or you can buy RO/DI water usually at your lfs)
-bucket(s) for mixing saltwater
-hydrometer
-fish food (flakes, pellets, frozen, or you can make your own! A variety is best)
-heater
Additional, but by no means required:
-phosphate media GFO & reactors
-chiller
-QT/hospital tank
Less equipment for filtration is usually going to mean more work with keeping water quality ideal
SW keeping is a crazy, epic experience with beautiful results