Currently, to gain remuneration for work as an instructor requires a minimum of a CPL and FI rating. This requires ground exams up to at least CPL level, a CPL training course, and flying instructor course (FIC). This assuming you already hold a PPL, night rating and have (if memory serves) 150 hours.
As for the eyesight requirements, I have no idea. However you would be advised to get a Class 1 medical before spending any money on flight training.
Regards,
C.C.