Basically, under BRCGS, if you are approving a supplier they need to be:
1. Certificated to a GFSI benchmarked auditing scheme
2. If not, a valid third party audit report is required (with some additional requirements)
3. if not that, and you (the supplier) are considered to be low risk (by the company approving you), you need to provide an approval questionnaire supplied by the company approving you. (which will have it's own requirements as dictated by BRCGS).
As the person that does risk assessments and approvals for manufactures and materials for my company, I'm certainly going to ask for a GFSI cert.
While not required by BRCGS, I am also going to ask for, at a minimum, a letter of guarantee, a certificate of liability insurance, a verification of your registration with the FDA (if applicable).
As far as the material I am going to purchase from you (understanding that you are in packaging, so some if these may not apply), I am going to want a product spec, a Kosher cert, a nutritional statement, a SDS, an allergen declaration, etc.
For packaging suppliers, I am probably going to ask for a product spec, a document that states that the packaging material is food grade, a document that specifies the composition of the packaging and probably a migration analysis for the packaging material.
Marshall