Like I said, you can build it into something you want later.

Then just get a dorans ring if you are worried about running out of mana. Buying an item you dont ever use is terrible.

She needs AP ASAP! The Meki Pendant is just to help Cass to spam her spells in her early game without having to worry about leaving the lane to recharge or having enough mana to finish off the enemy. Like I said, after the core build, it is up to the player. You can build the pendant into a more advanced item later on if you want.

With Cass's passive and the mana regen masteries, seals, and pendant, you can literally stay in lane spamming Q indefinitely with the mana bar about 75-80% full the entire time (as long as you don't level up Q of course). This leaves a lot of mana for you to spam all of your spells when it's time to go out on a full attack.

The reason why I go straight for AP earlier on rather than go for something like Tear of the Goddess is because Cass is naturally slow and squishy, which leaves her a sitting duck since she is played as a DPS. She needs as much power as she can get so that she can kill her enemies before they can finish her off.

i am not too sure as to why u wouldnt recommend Tear of the Goddess as a core item considering how you took Meki Pendant as we as the fact that Cassiopeia can quickly charge up the passive with her low cd spells.