How to manage and estimate unstructured requirements received from customers

Question

A lot of the times during the bidding phase of a project I receive a software system's requirements from our potential customers in a very unstructured format from various sources [email, word documents, excel]. It is usually a bunch of "product development" guys from the customer's side who come up with these "proposed solutions" to the business problems they have. While they are the experts at the business domain, a lot of the times they don't have the solutions right.

This results in

• multiple versions of the same requirement
• mixing up of two requirements into one
• a few versions of the requirement later down the line, the requirements which were combined together get separated out again, each taking with it some of the new additions

How do you work with such requirements coming in and sort them out into proper use cases and before development begins? What tools can we use to track a particular requirement's history, from the first time it was conceived till the time it gets crystallized into a proper use case? Estimating work against requirements received in such a fashion is a nightmare which ends up in making mistakes in understanding the requirement correctly and estimating the effort against it correctly.

Once we win the project, then the customers have given some more thought to their requirements and have been able to articulate it properly. What happens in this case is that some functionality gets dropped, some enhanced, some take a whole new turn. This basically can nullify some of the work item's estimates that were made before the project was won. I would be interested in knowing if there is any system by which we can build a tree of a particular requirement and how each branch resulted in a different estimate.

Any tips, tools, tricks to make this activity more manageable? I'm just trying to get some insights from someone more experienced than I am in requirements management and effort estimation.