Can I use a phone that I bought in India?

Thanks for the wonderful and useful information here.
I am planning on going to Guandong for a week.
1. Can I buy a SIM card from China - and use it in a Nokia phone I buy from India?
2. What are the acceptable forms of ID?
3. In Guangdong which is the best carrier - China Unicom?
Thanks so much for the help.