If you build Morello's on Cho'Gath

Your babies will have no butt.

You can certainly get the AP from another item around that cost. You definitely don't need the mana regen as you gain mana through kills. You definitely don't need the CDR since the first thing you'll be ult'ing is a minion unless you need to secure a kill. Grevious Wounds has so low value since your ult does true damage.

Alternatives:

Abyssal Scepter for 2440. You get AP. You have MR. Your opponent have MR reduction which means your Q, W, and spikes do more damage. Plus, it's a good first step to tank build and helps your team in team fight by lowering enemies MR.

ROA for 2800. You get 60 AP initially. Your tank build grows over time. Your hp/mana regen whenever you level on top of your passive. Best first item.

Rylai's Crystal Scepter for 2900. You get AP. You get HP. Now you'll land your Q every time after you cast W.

Core Build: If enemy team is CC heavy. If enemy team is squishy. If enemy team is AD heavy. Rush this before anything. Follow by this. Get this next. Because you'll have ROA already, you can sustain against your AD opponent fairly well. Good armor item to get because it has AP in it. You would think you need heavy armor versus AD heavy team, but this will keep you well alive. against all AP team, as this will shred the target (or multiple) that gets hit with your Q. This on top of your ROA makes you godly. This will protect your stacks if you die.