Know-How Exchange : E-Marketing

We are courting a new client who is starting a clothing line to provide online marketing services such as development and management of Facebook, Twitter, and YouTube accounts, content planning, posting and recommendations, email blasts, and even creating an e-commerce site. We have experience creating websites, blogs, emails, etc. but this would be our first client where we are mainly focused around creating some social buzz and ongoing flow of information, thinking on our feet to hopefully create some content that is innovative and really resonates to their market. We are more accustomed to very start-to-finish project oriented agreements.

We need some help figuring out the best way to create an agreement for an engagement of this nature. I know this is a very open-ended question, but I'm not sure where to start - a retainer? an SLA? project quote? How do we manage the expectations and scope of the work? Any insight broad or narrow based on your experience with this kind of work would be much appreciated - thanks!