Live sand will not buffer the ph at all. You can get livesand for the bacteria boost it will have, but inert sand will be fine and become live over time. As far as depth, that all depends what you want. If you want it to act as filtration, then you need about 4 inches. If you just want substrate on the bottom, an inch or two is fine. You should use ro
water. Not tap. The salt mix should do your buffering for you. If not, cross that bridge when you get to it.
The more liverock you can get to start with, the better off you will be.