May a congregation appoint an unmarried man to serve as the Chazan?
This question is addressed in the work Orchot Chayim (by Rabbi Aharon Hakohen of Lunel, France-Spain, 1280-1330), who records the practice to appoint only married men to serve as Chazan. This practice is based on the Halacha requiring that a Kohen Gadol be married. Since a Chazan petitions God for forgiveness on behalf of the people like the Kohen Gadol, he, too, must be married so that he is free from sinful thoughts.
Accordingly, the Rama (Rabbi Moshe Isserless, Poland, 1525-1572), in his glosses to the laws of Rosh Hashanah (581:1), writes that a congregation should preferably select somebody who is married to serve as Chazan. He adds, however, that this is but a preference; strictly speaking, anybody accepted by the congregation is suitable for the role of Chazan. Furthermore, the Mishna Berura (by Rabbi Yisrael Meir Kagan, or the "Chafetz Chayim," Lithuania, 1835-1933) rules that an unmarried man who is God-fearing and studies Torah takes precedence over a married man who is ignorant of Torah. The preference for a married individual applies only if both candidates are God-fearing, committed Jews; if, however, only one of the two candidates has these credentials, then he takes precedence even if he is unmarried and the other candidate is married.
It should be emphasized that this preference for a married man over an unmarried man applies all year round, and not only on the High Holidays.
The Kaf Ha'chayim (53:16) makes several comments concerning the qualifications required of a Chazan. He begins by discussing the etymology of the word "Chazan," and he claims that it evolves from the Aramaic word for "seeing." The Chazan must look in the Siddur in order to ensure that he pronounces all the words correctly, and for this reason he is called the "Chazan." The Kaf Ha'chayim proceeds to cite the verse in Shir Ha'shirim (2:14), "Show me your appearance and let me hear your voice, for your voice is pleasant, and your appearance, comely." He explains that God instructs us to first show Him our "appearance," to repent and appear before Him in a state of spiritual purity, before we sound our voices in prayer. This is particularly true of a Chazan, who represents the congregation, and must therefore ensure to be clean of sin before leading the services. The Kaf Ha'chayim notes that speaking in between the silent Amida and the repetition of the Shemona Esrei constitutes a violation of Halacha. Therefore, it is wholly inappropriate for a Chazan to speak after completing his silent Amida before beginning the repetition. After committing a transgression, how can he then come before God as the congregation's representative?
Summary: A congregation should preferably appoint specifically a married man to serve as Chazan; if, however, an unmarried candidate is particularly God-fearing and committed to Torah, then he takes precedence over a married candidate who does not have these credentials. In general, the person leading the services must ensure to be free of sin, so that he is worthy of representing the congregation in prayer before the Almighty.
