Lots of questions. I will say a T&G floor is not typically a DIY project. However I get that you want it done right and are willing to learn.
I will also say that a real T&G floor is not practical in every home as a retrofit project.
Subfloor is plywood, good, T&G? How thick?
Size of the joists? 24"O.C. is ok if the joists are sized properly, the subfloor should be 3/4" T&G.
How big is the area? 1/2" out of level is not a big deal in a large room, different story in a bedroom
Realistically forget level, the goal is to make it flat.
Is the new floor to be laid parallel or perpendicular to the joists?
Is the floor end matched in random lengths?
I would make sure the subfloor is screwed down.
I would be concerned with fasteners holding in plywood.
Do you have many transitions to deal with?
Any stairs?
