Does a chazan for a ta’anis have different requirements than a chazan for the rest of the year?

If a person has a tarnished reputation, even if only because of sins committed in his youth for which he has since done teshuvah, he should not be appointed as shaliach tzibur. It is questionable whether an occasional sinner with a less-than-ideal reputation should also be disqualified. The poskim also disagree about whether this restriction applies to any chazan or only one who wants to serve during a ta’anis tzibur or the Yamim Noraim. If he only sinned occasionally, it is permissible to be lenient throughout the rest of the year.

סעיף ד-ה ס"ק טז,כ,כא,כב וביה"ל דמכל מקום

Can one be demoted from his position as chazan because of rumors about him?

If there are reports about a person who has already been appointed chazan suggesting that he sinned in the past, he may retain his position. Contemporary poskim explain that this discussion about disqualifying a person with a checkered past only applies to one who was raised with Torah and mitzvos and then sinned. One who was raised in a secular environment and became a ba’al teshuvah is viewed as having a clean slate, and he may serve as shaliach tzibur.

ס"ק טו; ביאורים ומוספים דרשו, 3

Which nusach must the chazan use for his quiet Shemoneh Esrei?

A chazan must be fluent enough in the pesukim of Tanach to recite the pesukim that are woven through the davening. He must also daven in the nusach of the tzibur. Many poskim allow the chazan to daven in his own nusach during the quiet Shemoneh Esrei. He should take note of the differences in nusach so that he will not be confused during chazoras hashatz. Others say that the quiet Shemoneh Esrei is a preparation for chazoras hashatz and should be davened in the same nusach planned for chazoras hashatz.

ביאורים ומוספים דרשו, 27

• It is best for Yishtabach--which is the after-brocha of Pesukei D’zimrah--to be said upon the completion of Pesukei D’zimrah. It is also appropriate that there should not be a break between Yishtabach and the Kaddish which is said afterwards.

• A person who only got a tallis and tefillin when he was in the middle of Pesukei D’zimrah should pause at the end of a perek, recite the brochos, and put them on. The customary pesukim should not be said.

• The shaliach tzibur should be an honorable, righteous person with a long-established sterling reputation. The poskim disagree about whether it is appropriate to appoint a person with an obvious physical defect to be the chazan. Some encourage this, since the prayers of a broken-hearted person do not go unnoticed. 

• Can a teenager be appointed as chazan?

• When should a bar mitzvah boy not serve as chazan?

• Can a bar mitzvah boy be the chazan for Ma’ariv on his birthday?