Almost nobody wrote software that explicitly requires a 68040 processor. The 68040 requirement probably means that performance will be so bad with a 68030 that the manufacturers would not support it. As you've noted, a 24 bit graphics card is essential which more or less rules out a 68K PowerBook, but a stock 68030 IIci with decent graphics would no doubt work S L O W L Y.

If you can generate the scans on your PB 1400, why not use it for that purpose, then transfer the images to a more modern machine for manipulation?