Us e this to calc sand depth - Sand Bed Calculator
My opinion is that you want to do the sump with a refugium built in. Do the DSB
there and not in your display tank. The rationale is that within a DSB
the bottom layers develop toxicity over time. It's not a problem if left undisturbed so you don't want to have anything that can borrow and stir up the deepest layers. If you do your DSB
in the sump, you can close it off from getting into the DT if disturbed.
If you just do like 2" in your DT, you can avoid the problems within the display tank.
I would go for the aragonite based sand which you can get in really small diameters - < .5mm