Does a plastic surgeon have the tools to diagnose nasal obstruction during first consultation or ENT required?

I would recommend consultation with a board certified facial plastic surgeon who typically has a board certification in ENT surgery as well in order to diagnose any concomitant nasal obstruction or other breathing or sinus issues that might be present as it would be best to address all of your concerns at the same time including the appearance of your nose.

I'm not sure I understand the question.  Are you planning on having cosmetic rhinoplasty performed by a general plastic surgeon, but want to be evaluated for internal nasal issues prior to having surgery?  Otherwise, I don't know why you would need a "letter" to bring to the plastic surgeon.  Without sounding like a snob, I will just say that if any surgeon is to operate on your nose for cosmetic reasons, but they don't understand or have the "tools" to evaluate the inner workings of the nose, ie septum, turbinates, nasal valves, etc., then they should not be operating on your nose.  With bias, the solution to your quandary is to see a facial plastic surgeon who specializes in rhinoplasty.  This is a surgeon who is trained in ENT and plastic surgery of the face.  This would allow 1 stop shopping.  Good luck.

 Best to seek out a double board certified ENT/ facial plastic surgeon who can evaluate the internal functional issues in the nose and address them Medically and surgically. Functional nasal surgery and cosmetic nasal surgery can be performed simultaneously  under one anesthesia with one recovery period. A CAT scan of the sinuses is going to be required to make the determination about nasal polyps and chronic  sinusitis.