into most ap matchups you have prio until level 3 because of your auto attack damage (you always get 2 first cause of chimes too), you have bad wave clear early so be sure not to fall under tower. managing chimes and cs is difficult if your in mid game and everyone is stealing cs honestly buy a support item why not free gold. first back get a sheen which will build into lich bane later but for now will give you some more damage since early you wont have a lot of meeps making your damage limited with ap.

As for why there are so many choices for last 2 items, if you need defensive agaisnt ap wits end or banshees vail are good, against defensive ad zhonyas is good against both demonic or cosmic give good hp and shadowflame and nashors are super greedy but can work for lots of damage. agaisnt a lot of tanks definetly demonic, I'd say unless theres at leasy 4 ap go zhonyas, super busted item. spellshield is okay but you get good stats from wits end. to recap best is probably wits and zhonyas, if they dont have much ad or ap swap as necessary, high tank go demonic, high ad but you dont want demonic go cosmic drive. high ap go wits and banshees.