OK, as a professional in the Garage door industry, 44 years plus, here is what you can do:
you have 14-6 under you current ceiling, assuming you do not want to deal with the trusses, you are somewhat limited. 12-0 wide opening should be adequate, I'd probably go for a 13-0 door, that will give you 18" of clearance, plenty for standard 15" radius tracks and a trolley operator. you could probably squeeze in a 13-3 door. 4 21 " sections gets you to 7 feet and will match your other doors, 4 24" sections gets you to 13-0. To reach 13-3 only stack 3 21's at the bottom. If you have windows in the 3rd section of the other two doors, they will line up fine.
For an opener you have a couple issues, if the building is in anyway attached to your home you have to use a residential operator. There are decent units that will work, but you need to use one designed for Carriage House doors. If the building is separated you can use a commercial operator.
2016 Tiffin Allegro Red 38QBA
2008 Jeep Liberty (aka FireToad)