I have one suggestion for you. Storage Buildings are never big enough and once you get all your stuff in them they are underestimated in size.
I built a 12 x 16 myself from a design I saw at a local building center. I did some extra stuff that I encorporated into my building like extra tall and wide doors, double doors. I also used paino hinges for the doors and have never had any trouble with them.
Well one year later, this shed is too small.
I have my eye on a new design used down here and I really like it. It is shaped like a barn in that the roof has three angles. Its more rounded off if you will and this larger roof or lack of sever down angles of a single pitched roof allows much more storage in the roof area for what ever.
I even saw one with a attic type drop down stair case.
There are so many customized amenities that you can add to fit your own needs. I wanted to add windows, but I was told that a window is a good place to gain access for a thief.