Why did Chiram send to David?
'Rashi', from Bereishis Rabah 85:4: He always loved Shevet Yehudah. [This is] Chirah ha'Adulami, Yehudah's friend.
Malbim (Shmuel II, 5:11): It was to make a Bris with him and build a house for him. Just like David was king over all Yisrael, also nations that he did not know will serve him!
In Sefer Shmuel II, Perek 5, this episode precedes bringing the Aron from Kiryas Ye'arim. Why is it taught here afterwards?
Malbim: There, it wants to teach together both episodes of bringing the Aron. Here, Ezra revealed that the first episode was immediately after Yisrael declared him king. At that time he conquered Yerushalayim.
What are "v'Charashei Kir v'Charashei Etz"?
'Rashi': They are professionals. Radak (Shmuel II, 5:11, citing Targum Yonasan there): They are professional wood cutters and stone quarriers and expert architects to build walls.
