Tough room - what is your budget (low-end $ and top-end $)
How do you see your budget allocated - AV equipment, speakers, electrical, etc.
You can always use what you have now (speakers, AVR, etc.) and upgrade later leaving more available budget for what you do not have to get it "right" in the long term. This way you can also gain a better understanding of what your needs are and what sounds nice, but may never use.
Looking at the room, I'm thinking you may be stuck using in-ceiling speakers for your surrounds if you do not want the speakers sitting in the middle of the floor. You should have no problem using your front three mains initially to get set-up. You can upgrade if you are not happy with them - bu this depends on budget
My initial thought would be that your 1st row of seating would be approx 11 - 12' from the screen wall. I'm not sure you will have much room left to add a second row, but you may be able to add a single chair or two along the sides/back of the 1st row. You may want to vist the dedicated theater forum a bit to gather more data on screen size and seating distance.