How do I split a model perfectly in 1/2 and mirror it?

All said in the title

or is this even possible?

B for box select. Select half the model’s faces. X, delete Faces. Then add a mirror modifier in Editing Context>Modifier Panel. Or you can duplicate (shift D) and press M to mirror an actual editable mesh.

The mirror modifier is better solution, when you’ll need to have each side different, you just apply it. But make sure the object’s center is in middle, or you get crossed faces or they will be away.