Roa Deathcap and seraphs gives a minimum of 481 ap

Using poppy, who has the lowest base mana at level 18 (745), you would get 481 ap with this build if we take a ap based champion that could benefit well from this build, you would get at 911 mana this gives you 485 ap this is no runes, no masteries, no other ap items. This build, if you want to go raw damage, is incredibly strong. it doesnt give any utiliy, but you still have 2 item slots (assuming you get boots, if not then 3). The mana gives you the power to spam your spells all game, and the roa plus seraphs active gives you a good amount of survivability. honestly this build is so powerful, it can go on almost all ap champions. last 2 slots can be a cdr item and a utility item and you could be set.