Is it safe to use kali linux in virtual box in windows7 and is kali linux good for daily purposes?

Answer 1

yes why not , but you can use some features .

ie:
you have ​to buy wificard for searching wifi.

Answer 2

you can use it in a virtual machine but you will find out that some Features are only working in persistent mode.
And it is a bit hard to use for daily purposes. but you can.
Kalis purpose is to do Penetration testing.

Answer 3

yes u must buy a wifi card which is easily available on amazon to penetrate or hack nearby wifi signals