Why did he say "Yamim Ba'im"?
Malbim: You said that they are from afar, so there is no concern lest they come to make war. Through showing them your treasures, they will come here regularly, and in the end, take everything!
Why was Chizkiyah punished?
Rashi (Melachim II, 20:14): Refer to 39:3:1:1. Also, he rejoiced over the Sheluchim and fed them on his table.
Radak (Melachim II, 20:13): He became haughty that the Sheluchim came to see him from a far place, and that he succeeded in all his ways - "Gavah Libo" (Divrei ha'Yamim II, 32:25). He said that they came for his honor. He should have said that they came for Hashem's honor! They came "Lidrosh ha'Mofes" (Divrei ha'Yamim II, 32:21) - to investigate the wonder!
Malbim: He erred in two ways. (a) Since they came as spies, he should not have shown then his storehouses and wealth. They put this in their hearts, and came when they had the opportunity to take this wealth - especially since it was taken from Machaneh Ashur after it was stricken. Kings used to hide their wealth from their neighbors, to avoid envy! (b) A king is commanded not to increase his treasury [more than needed] or become haughty.
Why will nothing remain?
Rashi: This is Midah k'Neged Midah, for there is nothing that he did not show them (verse 15).
