One thing I will add here. If you are going for a steel exterior door, dont just tin the bottom 12 inches, you dont want short tin or cut tin. I would just steel panel the whole exterior, it honestly would not take terribly much longer than just doing the bottom little bit. If your interior framing is correctly spaced and you know your layout you can stack your sheets and measure out and drill perfect pilot holes for all the fastening screws so it goes up super fast and looks perfect when complete.

When the shingles finally go bad, then lay a sheet metal roof over the shingle and then you have mecha shed.