You can repair the front lip and bumper via plastic reconditioning for $700-900. Even if you buy them new from Nissan, you will still need to repaint them...
Painting of the front bar plus lip and the RHS sideskirt shouldn't cost more than $800-1,000 to repaint.
Your exhaust just needs to be repaired. No need to replace unless it is completely mangled (which doesn't apear to be the case from what I see in your pics).
Not too sure if the carbon diffuser can be repaired. If not, then surely you can source one for less than $4,000. Like I said, I can help with any and all of those parts. PM me if you need any help.