Dear Lebron (if I may),
The TEI is not a product for sale, so I do not understand your question about purchasing a "package". The TEI certainly does provide guidance on document data modeling for cultural heritage projects, and it is supported by a number of databases that handle XML structures, such as the free and open source eXist-dB. 

TEI XML may be stored and indexed and retrieved in a database, but it is not restricted to this use. Using TEI is simply a decision, and perhaps it is called for when information is more complex than you can describe in columns. Why would people ever turn to the TEI to represent data? Let me present you an example application of the TEI that does not involve database software and that expresses its TEI semantics directly in the web browser. 

The project involves study of an ancient manuscript written in a language that is unknown today:
* The manuscript contains images of plants and animals and people that can be categorized, studied, conjectured about. It also contains glyphs that people have been attempting to study.
* The organization of the pages in the manuscript is a little unusual: We can see a regular pattern of how the vellum was folded and cut and organized into bundles, but in some places, leaves have been torn out, and there is some damage on some of the surfaces.
* There is evidence of multiple "hands" writing on the page surfaces. 
* The page surfaces themselves contain material evidence of how the manuscript was made and how it was bound and shared over time among collectors. Scholars are attempting to trace where it originated. 

A feature of TEI XML for scholars is that it is designed to express what is known and recognized, and also to identify precisely what is unknown. It is exemplary and powerful for organizing metadata (data about the data) in either "stand-off" or "inline" ways as helpful: with annotations running within the markup structures pinned to external reference structures. Much of this will feel like working with a database, but it is not restricted to database technologies. The case we describe here is one that has lately involved migration:
* from one format of ASCII representation in a nonstandard text format used by the manuscript researchers, and 
* into the TEI, for the purpose of organizing the information, representing the document, and its metadata in a standard way that is recognized and workable for scholars around the world.