I'm not sure what "none too cheap" equates to, but 14" rough-in toilets are readily available for around $300. Any decent plumbing supply will be able to order 'em for you, and/or Google "14" rough-in toilets".
Check out this link
for some other possible alternatives, for example the Toto Uni-fit models link near the bottom of the page. This adapter has 10", 12", and 14" capability.
To answer your original question however, if you are OK with the extra clearance in back of the tank, the 12" will work just fine.