a DSB is 4 through 6 inches from what I have read and seen. and it really depends on what the person wants in my tank I kinda have a mix about 3 1/2 inches. If your going fish only with live rock you dont need sand at all but most ppl like it for the color and if you want it it dont have to be very deep.
deep sand bed
live sand of usually 4 or so inches. it must be seeded with various little live critters to keep it stirred and healthy. You can get a cup or so from the LFS tank or from a friend. Live sand in a bag won't do.
The reason to have a DSB is to lower nitrates.
crushed coral usually leads raised nitrates and more of a hassel to clean. usually with a dsb you dont have to clean. a good clean up crew and your developed dsb should do great. the dsb also helps filter your system.
It is good to go with a DSB but if dont want to cause of price I have seen of ppl using half crushed coral and half live sand. But the crushed coral will work its way up through the live sand "eventually". Or going and buying aragonite or base sand "it isnt alive" you can get it pretty cheap I got mine in 5lb bags for $3 a bag and then buy a good sized bag of live sand around 20lbs or more depending on how big your tank is and then dump it on top and get some seeded live sand from a friend. I got some from a guy here in missouri he gave me soem shells with coralline algea on them and some sand from his tank I just paid shipping $6. There are alot of cheapers way to do it you just have to look and dont jump the gun and buy what ever you find first.
I prefer a 2 inch sandbed in the display and a remote DSB in a refugium, but thats just my preference.
I would at least fill with sand to the top of the viewing panel just above the (normally) plastic brace or border of the botom of the aquarium.
Thomas