There is a Mennonite business here that builds sheds and can deliver at least a 12x16' one on a truck, and nicely made.

If you have a place like that locally, ask about repossessed ones. They may be cheaper than buying the materials and doing it yourself. All you would (likely) have to do is insulate it and add the bits you want.

But don't forget about access, which is my problem. I couldn't get a 12' wide shed through a <10' gate.

Sue