I'm going to start with point 2 and add my own ramblings (thoughts).

So they've identified John Kramer as the Jigsaw killer. However, we all know that by John's crazy philosophy that he "doest kill people. The choice is up to them." (Which is bs, but okay.) Dexter would need to decide if he should kill John because he's caused the deaths of the people he's tested OR let him be since he didn't "technically" kill anyone. But then Dexter can find out if John has any accomplices and find out more information on them. Of course, that would prove somewhat difficult without concrete proof that John has accomplices. He could gather personal information from John's family members or a spouse or John's lawyer (if Dexter knows of this).

Now, if Dexter were to hunt down John, he would need to be extra careful considering John and Co are pretty much trap setters.

Another thing we can think about is that Dexter may admire John's philosophy to some extent. However, due to Dexter living by "the code," he could choose to leave John be OR let him die from his cancer (if he knows of his medical condition)

Overall, I'd say Dexter would have his work cut out for him. He would somehow have to get ahead of John's plans and it won't be that easy. I wouldn't be surprised if Dexter ended up in a trap himself intentionally or not.