Yes, I think it is a idea to have the DICOM image type code available in ITK without duplication, but it should not be a blocker to move forward. Could an issue please be created here:
GitHub - KitwareMedical/ITKDICOM: Better support for DICOM in ITK.
that points to the appropriate code as a reference?
The overall plan sounds good
Worth noting, @pieper did some digging the other week and found these DICOM datasets from GDCM:
Grassroots DICOM / Gdcmdata / [a0bba5]
These are a good resource for regression testing.