Is it brand strategy development or brand strategy implementation that you seek? Approach and related services (agencies/consultants) will differ accordingly.
Strategic considerations include validation, clarification and amplification of master brand ‘promise’, making sure to define how it manifests as a complete brand experience, and the framework for leveraging that master brand throughout the enterprise (brand architecture, rationale for sub-brands and product names, etc.).
Considerations for implementation are different in that they are informed by the above brand strategy and include identity development (tone, look, logo, etc.), actual sub-brand and product naming, internal training to drive desired brand experience, etc. Implementation can also begin to address the ‘go-to-market’ aspects which include brand promotion and rollout of new brand elements across all facets.
Given these definitions, my recommendation is to be clear on your objectives first. Then the RFP is fairly straight forward. Being clear on what you want is the most important element of the RFP.