Sould the person acting as a prayer leader be Shomer Shabbat?

The Chazan that leads the Tfilah should be a man that represents the community before Hashem and as so should be an appropriate person of good characteristics. In any case he mustn't be one that doesn't believe in G-d or the Torah. (Mishna Brura 126; 2) Similarly he should be a Jew that keeps Shabbat according to Halacha. (Zichron Yehuda OC 6, Yalkut Yosef) In cases of great need the local rabbinic authority should be consulted.