If your going AP you might wanna get some Sorcerer's ShoesXD

teemo is very squishy, getting some kind of defense item is crucial imo since he can easily be bursted by any ap nuke. try evolving the build, every game might not go as you thought, what can you do to prevent it from getting worse? expand your build a bit more. + teleport with teemo's bombs is convenient but dont required. magic penetration marks and standard ap quintessences (+4.95ap) works better earlier on and late game.