So you want to play an AP Champion...
1 thing that most Ap champs should start start is the
or the
(only if youre a marksman Ap champ) and the health pots.
This is where you need to make a decision is you are ahead
(
)
(
)
(
or
))
Depending the Ap champ you rush these
if you are behind don't be afraid to get your boots or an extra dorans and play passive or communicate ganks if they want too jg are NOT obligated to gank you.
for my 2nd item it is mostly 
3rd now depends again
I get a
or 
other Ap champs
are examples that use ap differently.
does damn that scales with his mana so rod of ages is a must
is health based and needs lifesteal or spellvamp which is best using

, which is currently one of the more popular items to get on AP champs.
Many champs also get
or
for mana regen.(though none of the ones you listed are mana intensive...)