Based on the information above, what I have in mind is a mix of sandstone, limestone and reef rock.
I would combine the plans similar to what Anthony Calfo describes as "centered single seamont" on the bottom and "two seamonts, a large and a small" on top of the single seamont (page 15 Reef Invertebrates).
The single seamont would be made of the heavier sandstone rock and built up 6" to 9" high and would occupy the entire footprint of the tank except for a 4" to 6" clear perimeter around the tank. Do not have a source for clean sandstone rock that can be used in reef tanks and will require some research. However, there are plenty sources for Cichlid tanks.
On top of this, I would use limestone rock to create the two seamont feature. The total reef height would be about 18" high. The source for the rock would be any one of the Gulf of Mexico aquacultured rock sources, for example
Tampa Bay Saltwater or
Florida Live Rock. Both of these sources use limestone rock for the aquaculture process so perfect. As a bonus, I will get a good source for gulf coral, sponge and algae, especially coralline.
In the areas between the two seamonts, on top of the base rock, and around the perimeter i will use some dead coral live rock,
such as this from etropicals, to build up a "drowned reef" structure.
The tank will incorporate a 4" DSB around and in between the base rock to hold everything in.