First accepted offer

[#1]

I accepted a job offer they sent me information for a background check and a blank W9 to provide them. Is that okay? I thought everything will go through upwork?

[#2]

It is fine if they want a background check and you are comfortable as well, but try not to give up any contact information without having a contract on upwork. (Particularly this is a violation upWork terms and conditions). 

Regards 

Haseeb Ur Rehman. 

[#3]

No, you do not need to do a W9 for them. You are being paid through Upwork.  Upwork clients do not issue 1099s to you.