The following links contain information about selection criteria for digitization projects that may be helpful:
Handbook for Digital Projects: Selection of Materials for Scanning - http://nedcc.org/oldnedccsite/digital/iv.htm
Selecting Research Collections for Digitization by Dan Hazen, Jeffrey Horrell, Jan Merrill-Oldham (CLIR) - http://www.clir.org/pubs/reports/hazen/pub74.html
Selection for Digitizing: A Decision-Making Matrix (from CLIR's Selecting Research Collections for Digitization) - http://www.clir.org/pubs/reports/hazen/matrix.html
The NINCH Guide to Good Practice in the Digital Representation and Management of Cultural Heritage Materials - http://www.nyu.edu/its/humanities/ninchguide/
Selecting Materials: An Iterative Process (from The NINCH Guide to Good Practice in the Digital Representation...) - http://www.nyu.edu/its/humanities/ninchguide/III/
Copyright Term and the Public Domain in the United States © 2004-8 Peter B. Hirtle. Last updated 10 January, 2008. - http://www.copyright.cornell.edu/public_domain/
The 3rd edition of A Framework of Guidance for Building Good Digital Collections (produced by the National Information Standards Organization (NISO) Framework Working Group, with the generous support of the Institute of Museum and Library Services (IMLS)) - http://www.niso.org/publications/rp/framework3.pdf