You have to figure the amount of water to be used then figure displacement,weight of tank,rocks,sand,equipment on and in tank then do the math.
Your tank dimentions givin give you an empty water volume of 36.36 gallons , saltwater weighs approximately 63.6# to 64.3# per cubic foot depending on salt content, there are approx 7.4805 gallons in a cubic foot, giving you 4.8606376 cubic feet of water.Thats with nothing else in the tank.
As far as the stand goes 3/4 " plywood or a 2x4 frame should be more than sufficient.