RoA as last item is stupid .

I actually don't know why this got a down vote so quickly without any explanation.

I personally love playing Cho' as an AP off-tank, makes the damage you do with your ulti quite beastly late game, catch that Trynd off guard before he can Endless Rage!

Anyways, other than the side chatter, I just have a few suggestions I'd like to make regarding the build.

First off, I'd like to say that I completely agree with your runes and skill order for Cho - as a full on AP ChoGath, the least you can do it be a little supportive in the lanes and the you'll be utilizing a good start with the mana regen / flat ap quints.

The only thing I can probably give some critique is probably the your item order. While rushing Rabadon's right after boots is great for mid game, the time it takes for you to build up 3.6k is a pretty long time to go without holding out for items, or even for 1.6k for Needlessly Large Rod.
Phever makes a good point in suggesting you rush something for survivability, even if its just Catalyst before Philosophers Stone. Speaking of which, I can't help but notice that you don't have an item for Philospher's stone to build into, while the initial health/mana regen is practically good - I feel like the majority of your items would be impossible to attain in a standard 40 minute game on the basis that all your items cost a hefty amount.

Though it seems that you have a different approach to items in your description section, I feel that that relying on the ultimate alone isn't enough to give you enough survivability in the lane early game.

A slightly different starting route I'd suggest would be: Regrowth Pendant > Health Pot > Boots of Swiftness (or Sorcerer's Shoes even) > Catalyst > Philosophers Stone / Tear of the Goddess > Blasting Wand > Blasting Wand > RoA / Archangels - this is only for early-mid game.

There are also a few items I feel that could be a great boost for the build. You could build up your Philosopher's Stone up to Shurelya's Riverie, which would make you more of a support but the reduced cooldowns wouldn't be too bad for anyone (not to mention you'd be a chasing machine with that active). Morello's Evil Tome is also a great (and cheap alternative) to some mana regen/cooldown/AP problems you might encounter mid game if you decide to sub out Philosophers stone, you won't benefit from the health regen but you could always use your passive with Vorpal Spikes! ^_^. Another great item I'd suggest is Will of the Ancients, not only do you get 80 AP (which is the same amount as the Abyssal Scepter, but cheaper), 30 AP of that is shared as an aura for your team mates (which is never a bad thing) - as well as the additional spell vamp, good for both you and any other mage on the team!

While it's true that your build will give you the highest amount of static AP for any AP Character (minus Mejai's of course, and your Phil's Stone), I can see it being very difficult to be able to achieve the same build on a consistent basis.
Hope this comment helps! If you need me to clarify anything I'd be more than glad to!

You need something for survivability even if its like a rylais but you need it early so that you dont die to fast everytime.