Hi, I'd like to help you on this. I have a vast enterprise developer/architect experience and am designing similar/related functionality on a day-to-day basis, which makes me a perfect fit for this requirement.
My bid includes a detailed discussion with you over phone/skype, reviewing existing documentation and system, agreeing key targets and concept, then delivering a MS-Word format document with the "business requirements" and "technical implementation" proposal, the latter one being a medium-level specification. By medium-level specification I mean that it sets key development (architecture) concepts to ensure a proper implementation ("make it work") avoiding any design flaws but leaves low-level details to the developer. I will also include generic requirements on coding standards, design patterns, unit testing etc as required by you.
Also I will be available later should you need me as well as I may bid on the implementation ;-)
I look forward to working with you!