The 900 is a good case. I recommended the 300 + 2 extra fans beacuse it is cheaper. I have the 300 with those fans and trust me there is plenty of blue glow.
Nope, there is already onboard Ethernet that is Gigabit (1000MBPS/10MBPS/100MBPS)
Just throw your old wire in there, and if you configured your router and stuff to be ultra secure, add its MAC Address into its utility and you should be on the internet then.