Kevin I built one a while back for a family friend that owns a security business, and the door he bought was for 8" cmu and was 60" x 83 with a 3 or 4 hour fire rating and the floor was a 12" deep slab with #4 rebar 8" on center the ceiling was a 10" pour over with rebar 8" on center, the door sandwiched the cmu wall with heavy steel bolts that fastened to the inside steel frame and the door had a approx 5-6" cavity both sides and top that had #5 rebar verts and horrizontal that was grouted solid when we poured the walls!
I think 12" cmu would be over kill unless your tying into another wall that your already using 12" block on, but if you are wanting over kill I would suggest using #5 or #6 double curtain rebar for added strength!
And your right a good door will be expensive and fairly permanent!
