Active Directory Structure

I am in the process of scoping the best structure for an Active Directory implementation.  I am being told we should stick with a single Forrest, a single tree and a single domain and use OCs to define location and user settings.

I am not knowledgeable enough about AD to know which questions to ask but I do have an understanding that the design decisions in this early phase is critical to future administration success.

We have 9 locations within a 30 mile radius and I am hoping to get a better understanding of the strategic factors we should be considering prior to settling on a AD design.

Are there limitations to using a Single, Forrest/Tree/Domain Structure?  If so what are they?

When setting up the Name Space, how important is it to define the structure initally?  Is it a simple process to make changes later?

Please Advise