Here is a link that can help you calculate how much sand you will need. You still need to use some common sense but it should get you close to a final decision.
I'm not a fan of the GARF system so take my advice in that light. I absolutely, strongly, discourage you from going through the hassle and heartbreak of building a plenum.
saltwaterfish has macros as well. (in addition to gulf coast)
floirda aqua farms has some also along with micro algae which is good for filter feeders and feeding thier rotifers. Both of which make excellent coral food when cultured.
they also have an nutrient solution (micro algae grow?) to keep the micro cultures growing.
What I recommend is you simply start the tank with macro algaes right from the start and then do the rest. Best to protect the macros in some kind of refugium if only a simple partition to the fish won't eat the macros.