I would suggest that you get the rock first so you can place the rock and then the sand. Otherwise, when you decide to get rid of the sand for something coarser, you will play heck getting it out of the tank without causing your rocks to shift.
Of course, you could always go with a coarser substrate or even NO substrate right from the beginning, lol.
I have about 120 pounds of sand in my 90, maybe more and it is only 1/2 to 3/4" above the bottom band. So I am thinking that 150 may not be quite enough. I have some 3/4" HDPE sheeting in route which will allow me to ditch the sandbed and put down the HPDE, stack my rock and then put down some Caribsea Seaflor Special Reef Grade. That will allow me to point my powerheads toward the subtrate and rock and keep more of the detritus in suspension for the skimmer to take out for me. That means less siphoning which is a very good thing.