I'm working in a medium-size organization faced with several data
gathering and reporting systems where data definitions and terminology
have not followed common taxonomy standards across the organization. 
These independent systems range in size and complexity from relatively
small Access databases and Excel based solutions to very large SQL
databases.

I've been tasked to develop a platform where all of the existing data
sources can be pulled together in a consolidated data warehouse to
measure key performance indicators across the enterprise.  I'm only
interested in the business approach and not the architectural
approach.

Question: Does anyone have the experience with these types of
projects? Could they direct me to any manuals or documentations
dealing with the methodologies on how to tackle this? Any tools on the
market?  This sounds to me like a classic problem.

I'm specifically interested in at least two things: 1) methodologies
on assessing the current practices and data requirements across the
organization
2) Documentation/methodologies to help write the business requirements
document, drive consensus among the stakeholders

Methodology would be "agile programming".   Very dynamic and
responsive to User needs/participation.   Has history and status to
persuade stakeholders.   And is fun.   Google and review the results.