I had a 77G salt water reef tank for a few years. Took it down cause I was moving lots and it wasn't convenient. Still have all the gear, gonna set up again one day.
First thing, decide if you want to do fish, or corals. This drastically changes your goals from day one
Lots of lighting. I had 2x48" actinic flourescents and a 175W Metal halide 6500K I think.
Sump for sure for filtration. I had a 1200GPH pump in mine. Don't waste money on a protein skimmer (lots of debate here) if you do regular water changes and keep basic nutrients up in the tank.
Something I never did, but will next time, get all your fish/corals from one area of world. Say Hawaii for example. Don't try to mix atlantic, pacific, and mediteranean stuff. There are different water qualities in different areas. Research it and get stuff that works with your setup.
I found after lots of time and money, it takes a year to have a saltwater tank running great with no more problems if you setup from scratch. A few months it will be running well, but wierd things can still happen. Mine stabilized completely after a year. I also fed nothing to my fish. My tank was self sustaining. Just did water changes and added a few key things for corals.