I got reef saver dry rock from brs. I like it because it's not harvested from a reef and it's completely dry rock which means no nasty hitchhikers. Also it's really easy to stack and make interesting scapes and is quite a bit cheaper than live rock.
Some people say a pound per gallon but I say whatever you prefer. Some people like less is more or some people stack it out of there tanks! It really comes down to what you're going to have in the tank and the look that you want.
I would say based on what live stock corals and fish. You need room for corals to grow and fish to swim. You also need enough hiding spots for fish to sleep so I figure you will buy some and then add more to it later. Weight depends on the rock so much. Farmed rock has a lot of life but really requires curing to remove unwanted pest and you may still get some. Dead live rock is not needed unless it has a lot of coralline algae on it. Dry rock will become live rock after a short time and you can always buy a couple pieces from your LFS that are cured live rock to increase diversity. Some pest like a mantis shrimp can be a pain, I got one and had to pull each rock one at a time until I found it in a crevice deep in the rock. I knew by the loud snapping sound and disappearing fish.