Am I not mistaken, is 필요하다 considered a "descriptive verb" (I think that's the term), similar to adjective, so there's only 필요한 거 and 필요할 거. So even if it says 필요한 거, it can very well be present tense.

Yes. It is from 한거에요 or 한 것이다. But it has error in this sentence. Spelling and spacing. But don’t care about spacing right now. Just focused on the SPELLING. First, I wanna talk about 에요. 에요 is incorrect. Wrong. Cuz 에요 is abbreviation of 이에요. when 이에 is abbreviated, it becomes 예. So 예요 is correct! (This part is difficult for Korean too lol ironic) Second, 한 is 하다 + -ㄴ. And there is some rules that when word is combined, some part (in this case, “다”) is out. Gone. I don;t know how to describe this in English. Sorry... (in Korean grammar, it called 탈락) If you have any Questions about other things.. email me: [email protected] or just leave a comment.