Since others spoke about requirements, I will speak to something else.
Depending on the scope of your user-base (e.g., number of employees, its initial planned use, etc.), you might want to consider deploying Windows SharePoint Services 3.0 (WSS 3.0). Instead of investing signficant resources in license purchases, WSS 3.0 is available at no cost assuming that this is only being used as an internal application. Of course, you would still need the infrastructure (e.g., server, storage, redundancy, SQL, etc.) to support it.
WSS 3.0 has about 75-80% of the functionality that MOSS has. If your program decides that WSS 3.0 is too limited for your needs, Microsoft has created a streamline process in moving your WSS 3.0 environment to MOSS. In summary, you would buy the MOSS licenses and “turn-on” the new features. This saves significant hassle in having to do custom code or other work to get the MOSS functionality.
My only other suggestion is in regards to your deployment strategy. I would strongly advise that some type of SharePoint governance process be adopted in advance of deployment. SharePoint’s best feature is its flexibility, but that can also be its biggest downfall. Users that have some technical ability can easily create sites, document libraries, and other types of web content. If this grows too quickly, you may find yourself struggling to link the content together (e.g., Rogue sites, groups, etc.). Additionally, you might also find that your storage availability will need to expand very quickly to meet your user’s needs. Finally, you might also find that you are duplicating storage efforts if user’s are saving files on shared network drives in addition to SharePoint.
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