I'm building my first bike, a 105 Soma Smoothie 58, but am stumped on the headset. I purchased a Columbus Tusk Air fork, which calls for an integrated headset to mate with the frame. But the frame is not configured for an integrated headset. From what I can gather from the interwebs, It appears that I can use an integrated or non-integrated headset, most people say it doesn't matter. Which type is best to use? What brand would you recommend, in a mid-range headset?

You have no choice. The frame determines the headset, not the fork. Some forks will look better with integrated, some with non-integrated, but the frame must have the type that it was designed for. Most likely you can tell just by looking. If the head tube is perfectly cylindrical, that will take a non-integrated headset. If the headtube is bulged at the top and bottom something like a gentle hour glass shape, that takes an integrated headset.