Buy the complete EV3 set or only the brick?
Since the NXT 2.0 was released in 2009 you must have a first generation NXT set then. As per this question the peripherials (motors, sensors and of course their connecting cables) remain compatible. So you could just get the central unit and use the already owned NXT components. Keep in mind though that all parts (motors, sensors) have since been redesigned, so the EV3 versions will likely work better, and don't forget that the EV3 set includes an infrared sensor as well, which was not present in the NXT complement.