Kirk,
First and foremost, WELCOME. From the sound of things, It seems that have the potential for a really great tank. Having converted from freshwater myself, only a few years ago, I'll try to get you going in the right direction, as much as I can.
If at all possible, since your tank is used, try to find out if the previous owner ever used and medications in it. Several fish meds contain copper. Copper is deadly to to many marine species. If you have found out that copper has been used post the question on what to do to fix it. Frankly, I don't know.
Make sure you have a really good idea of what type of Corals/Inverts/Fish you want to house. This will dictate you're lighting requirements and to some extent your filtration. It will also help with compatibility and same money (Replacing livestock/Upgrading)
Continue to do research. As "Broomer" says "Your tank - Your choice"
IMO, I would consider using a 4-6" Deep Sandbed (DSB) aragonite sand seeded with Live Sand (LS). 100+ lbs. Live Rock (LR). 3 250watt Metal halide bulbs (pendants, since you have the beam), Supplemented with several VHO or PC bulbs 300-400 watts worth). A sump/Refugium, not sure on size, but I'm thinking around 40-50 gallons. And, one heck of a protein skimmer, I'd consider "Berlin".
This is along the lines of the plan I have for the used 80 gal I just bought. Please don't take my opinion to be fact. Do your own research, and take your time. This BB has a wealth of information. I learn something new everyday. Use the "search" feature and look up threads on the things I've mentioned. There should be plenty. I hope this gets you going. Best of luck and keep us all posted.
Kevin
