Could find yourself a used enclosed trailer, and install a couple windows, Add some lights etc. insulate, and panel it. If you install fold up benches you could still use it as a trailer in the summer months. The nice thing is you have time to find one and can buy used windows etc as they come around and store them in it till you are ready to build. Just an idea. I bought one that i use for a storage building and keep it on property i camp on. I was going to turn it into a shanty but i ended up using a pontoon boat instead.