1)
Here it says that there is no number. Below (29:4) it counts the gold and silver!
1.
Malbim: It counts a Segulah in addition to what he saved without number.
2)
Why did he say "Kum va'Aseh vi'Yhi Hashem Imach"?
1.
Malbim: You are not lacking anything, just to do (build) it! Hashem should be with you, for "Im Hashem Lo Yivneh Vayis Shav Amlu Vonav" (Tehilim 127:1).
