God cannot have a body for a body is of matter and form. Matter and form are complimented by one other ingredient to make a body, which is the ability of change. Remember in the quinque viae (English, Five Proofs) that change was a proof of God, which made this “God” the necessary being for all other change in the world? That idea is important here because God cannot undergo change (otherwise He would require a thing before and more powerful than Himself) and it directly shows why he does not have a body, by cause (Latin, a priori) and not effect (Latin, a posteriori).
Essence and nature are used interchangeably as terms, and God is the same as His essence or nature. Essence is what a thing “is”, e.g., humans have a nature that is “universal”. God cannot be “in” Himself. That is, God does not have a body and so there is nothing to be in. However, God is just pure act, the prime mover, He is not like or akin to anything as a genus or species like offspring of another thing; He is who He is and so whatever He is, is in fact, His very nature. “I am who I am.” Creatures receive their essence when they are created, but God was never created and so He never received his existence, instead He is His existence. (Remember this going forward!)
An accident is an external thing which is associated with a thing. For example, a hat has black color, it has its shape, its size, its smell, its taste. All of these things are accidents. God has no accidents, obviously, because He does not have matter and form (Latin, hyle and morphe).
To study the Summa Theologiae, I highly suggest doing so with Verbum from Logos bible software. Verbum is a state-of-the-art library that connects thousands of resources so that you can study a subject with layers of depth, analysis, and understanding. If you’re new to the Summa, you really need companion material and it’s all available in the Verbum package. For example, each Question and Article in the Summa sources several other documents of the Magisterium, other philosophers, Church Fathers, etc., – and Verbum connects them all, so that in one click, you can explore and connect the Tradition of the Church easily and scholarly. I use Verbum everyday, no exaggeration, it’s that important to me. Learn more about Verbum here.