[Eugene]

WPA2 is stronger encryption and you should use it when available. You would only enable WPA if you have legacy devices that cannot auth via WPA2.

Allow setup over WAN means you can use AirPort Utility to access your basestation from the internet. You should leave if off for security purposes.

The only way to hide your WiFi network is to not broadcast your SSID, and that's not very effective from people snooping around.

5GHz is a higher frequency band with much greater throughput and it's the only way to get "AC" speeds with the new AirPort hardware. You should definitely turn it on. Whether you choose to hide the SSID is up to you, I'd just leave it on so that your devices can discover it more readily. Leave your channels on Automatic if you want, but if you experience packet loss or slow transfers, you might have to try different channels.

60mbit is a bit slow for any "N" or "AC" connection unless there's a pretty good distance between you and your AirPort basestation.