to build a dsb, you need about 4 to 6 inches of substrate, i really like ls, but it is very expensive to build a bed out of ls only, you cna use 1/4 ls and 3/4 dry sand, or use mostly dry and lr(the critters in the lr and bacteria will make the sand live, eventulaly), i used (i forget exactly how many bags) most dry aragonite(i feel aragonite is best, but silica or quartz seems to work too) and one bag of ls, plus alot of lr,
but the main thing is that it does not all have to be live, especially if you aree using lr
and bud answered your questions about salinity and the actual differences