1) Not sure about which level was in demo, but at least in the real game the first level was more or less tutorial, so the zombies didn't attack. But no worries, those will attack in future levels
2) Well, this is more like personal opinion, but IMO it the game doesn't look too bad. But for me, graphics aren't everything, but what the game really contains.
3) Hard to say, as I'm playing with laptop with, but at least CPU and RAM -wise you won't be able to use even near the top end graphics. I have double of what you have, and if I try to turn the highest graphics, the game gets really laggy.