Karthus - how to Overture?

Greetings! I've recently picked up Karthus, being the underrated champion dabbler that I am. Takes a fair bit of psychogames to make the most outta his duelling potential, but I love his behind-the-scenes impact on games. I also appreciate the confusion he brings to the lane - and to the game, - cause apparently Karthus isn't encountered too often these days. That being said, it feels easy to Requiem the victory out of the late game. However, it feels confusing to Overture into the laning phase. I have witnessed so many item paths, fully effective on Karthus one way or another. I will not post my own itemization - to avoid needless criticism. I come here asking, though: what are your thought on the most effective Karthus build? Mana and CDR vs tankiness? Extra DoT vs raw power? What do you think?

~CB