4 inches of sugar fine sand is good for a DSB. Some people, like me, like to keep a layer of special grade on top of thet to prevent sand blowing around but you won't need a lot of current in a SH tank. The sea grasses, such as turtle grass, need a deep bed, but not sand. They are a true plants and have roots. They need a mud bottom. A layer of sand can go over the mud. They get nutrients from their roots, and not from the water like macro algae do. Some of the caulerpa have long fronds that will work fine in a SH tank without going to the trouble of putting in special substrate. I know Richard is looking to put in Turtle Grass in his SH tank. You might want to check with him.

As for cooling, you may want to consider putting in several high velocity cooling fans instead of a chiller. You won't have to deal with the heat issues of MH in a SH tank and that may be all you need.