Because he has to get up close he needs health and resistances. His passive scales off health too. His Ult also complements building health on him by Reducing 20% of non-tower damage taken when in the circle.
I wouldn't buy something like Archangels on him.
If you wanted to go AP build, ROA would be the first one that comes to my mind. Builds from catalyst so you won't have mana problems. If you want to push waves and just spam spam spam then just go B after using all your mana. Or get blue buff.
In no real order.. Roa, Rylai's, deathcap, boots, abyssal scepter and zonya's. I think that'd be a good AP Maokai.
That's just how I would do it I think, but There are better tanky AP's.