you should add all the sand to a dry tank then cover with plastic bags and place a glass bowl in the center, fill tank by pouring water into the bowl. The bags will start to float, just leave them..... When full remove bowl and bags you will have almost clear water, no sand storm.
You can add rock before this just cover as much sand as possible and leave a place for the bowl.
Thats a lot of sand..... Why do you want a 6" sand bed? Have you seen a tank with a DSB
they look like crap... this is just me.... I have 3-4" in both my tanks and they are fine.
You also should mix your water before adding to the tank. I mix in a 35 gal
Base rock is ok but if your thinking about getting LR
you should get as much as you can right now. LR
is the best filtration going..