Shoftim

 PLACE OVER YOURSELVES A KING WHOM G-D YOUR L-RD SHALL CHOOSE, FROM AMONG YOUR BROTHERS  PLACE A KING.  YOU MAY NOT PLACE OVER YOURSELVES A FOREIGN MAN WHO IS NOT YOUR BROTHER (Devarim 17: 15).

If G-d will choose the king, why must the nation be commanded to select a Jew?

If a "prophet" declares that G-d has selected a non-Jew to be king, that alone is enough to judge him a false prophet (Rav Moshe Feinstein).

ONLY HE [the king] MUST NOT HAVE TOO MANY HORSES SO HE WILL NOT RETURN THE PEOPLE TO EGYPT IN ORDER TO INCREASE HORSE, FOR G-D SAID TO YOU "DO NOT AGAIN RETURN TO THIS WAY (17:16).

Why does the verse refer to "horses" in the plural but then say "horse" in the singular?

After one sins against the Torah, that behavior seems permissible in one's own eyes.  A king who falls to the temptation to acquire many horses may be willing to return to Egypt for even one horse (Chafetz Chaim).

HE MUST NOT HAVE TOO MANY WIVES, SO THAT HIS HEART NOT BE TURNED ASTRAY.  HE MUST NOT AMASS TOO MUCH GOLD AND SILVER.

IT SHALL BE WHEN HE SITS ON THE THRONE OF HIS KINGDOM, HE MUST WRITE FOR HIMSELF TWO COPIES OF THIS TORAH.  IT SHALL BE WITH HIM AND HE MUST READ IT HIS ENTIRE LIFE, SO THAT HE LEARNS TO FEAR THE L-RD HIS G-D, TO KEEP ALL THE WORDS OF THE THIS TORAH AND THESE DECREES TO DO THEM, SO THAT HE DOES NOT BECOME ARROGANT OVER HIS BROTHERS, AND NOT DEPART FROM THE COMMANDMENT RIGHT OR LEFT, SO THAT HE PROLONGS HIS YEARS OVER HIS KINGDOM (17:18-20).

The Torah is the cure for all of the king's character illnesses.  Too much money increases arrogance, but the Torah prevents arrogance.  Too many wives turns his heart from G-d, but Torah prevents such a turning.  If he trusts in too many horses he will die in battle, but Torah prolongs the years of his kingdom.

'Throne" is Kisey spelled Kaf-Samech-Alef.  This stands for Kesef-Soos-Isha, which means money, horse and woman.  If the king does not amass too much of these three things, he will merit to preserve his "throne." (Kli Yakar)