What's the best way to build a deck sub-structure?

I've been researching deck designs and drawings and came to the conclusion that people build deck sub-structures in many different ways. Of course, I want to know if there's a "best way" or if it's all personal preference. I want to build a deck that's attached to the concrete slab of my back porch/lanai. It would be 14 feet wide and 18 feet deep, running level with the back porch slab because the backyard slopes downward 28 inches in those 18 feet. The first 9 feet of substructure (going outward) would be sitting on the ground and the back half would be elevated. I'm also planning to build a railing 3 feet high on the back half of the deck. Here's my slew of questions regarding this project:

1. Should the beams on the back half of the deck sit on top of the posts or be bolted to the side of the posts? If the beams are bolted to the sides of the posts, then I could use the posts for the railing as well.
2. Since half of the beam/joist sub-structure sits on the ground, then could I use 2x8 for the beams and 2x6 for the joists? I've seen designs use 2x10 or even 4x10 beams, but my deck would only be semi-elevated.
3. Since the cost of composite deck boards is very high and I'm skilled in wood-finishing, I was thinking of buying either 1x6 or 2x6 pressure treated boards and finishing them myself for the deck surface. Is that a good idea? Would non-pressure treated 2x6 boards work as well?
4. Are those cheap molded concrete footings sold at Home Depot sufficient for my posts to sit on? I originally thought I should pour my own concrete footings, but I wonder if those would work. It would be a lot less work if they did. This would only apply to the back 3 posts for the elevated part of the deck.

Thanks for the help everyone.