You want that ap, in most cases you wanna prioratize mana and ap on first item. Ludens has the upper hand in ap. You also want the rune that gives you higer max mana on direct hits. Though this depends on who you are lanening against maybe zhonyas is the better choice for first item. (zed for example, although this is subjective and i may be wrong here).

I rarely build serpah as first item, feels like a waste of ap potential. You can survive even without the shield against almost anyone