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/