What is "k'Arba'im Elef"?
Radak: The Chof is of Shi'ur (about 40,000).
What is "Chalutzei Tzava"?
Malbim (12): They were Necheltzu (chosen) among their brothers. This was less than half the number of men of the proper age to go to war from these tribes.
Why does it say "Lifnei Hashem"?
Malbim: This was part of the stipulation. They went l'Shem Hashem, and did not rely on their strength.