IMO, the unit should be no more than 74 3/4" tall (meaning the slab of the door would be about 73 1/8"... not very tall), and that's provided the sill and head of the rough opening are perfectly level. If you plan on trimming the door with 2" wide #908 brickmould, the top brickmould will protrude roughly 1 1/4" beyond the top edge of the door jamb, so that's where I arrived at 74 3/4. (76 minus 1 1/4.)
As for the wall thickness, order the door with the maximum wall thickness that you can get. Ideally, the door jamb with extensions and the aluminum sill should be about 7" wide, the brickmould trim would make the full thickness around 8 1/4", meaning the exterior trim (and nose of your aluminum sill) will be about 1 3/4 inset from the exterior surface. Hopefully that would work with your cement pad. You surely want the interior of the door to be flush with the interior surface of the wall, and yes, it's nice if it can open beyond 90 degrees.
As far as sealing the door, the last few inches of the aluminum sill are hollow, and you can't seal that part very well at ALL. Your best seal under a sill usually ends where the jamb ends... on most doors that I have worked with anyway. They are solid until you get beyond the jamb then it becomes hollow under the sill. Sealing the front edge of the sill usually only creates a "dam" and works against you by trapping water under the sill.
Last edited by XSleeper; 11-17-2007 at 04:11 PM.