The first quality that makes Jonas the perfect candidate to be the Receiver of Memory is wisdom. Technically speaking, Jonas doesn't have this one . . . yet. It is the one that he can and will acquire with training.
The second quality that Jonas has is intelligence. Jonas has shown through his work at school that he is a top student.
The third quality is courage. This quality is similar to the wisdom quality. Jonas hasn't necessarily shown this quality a lot yet. But both the chief elder and the current Receiver "feel certain that you [Jonas] are brave."
The fourth quality is integrity. Jonas has shown himself to be a good rule follower. That doesn't mean Jonas is perfect, but Jonas has always been honest about his errors. He confesses them and accepts the consequences.
Technically, there is a 5th characteristic. That characteristic is "the capacity to see beyond." It's why Jonas can see things in color. The current receiver also already confirmed that Jonas has this quality.