Bob Henry really knows his stuff!
I haven't build many sheds, but I did, once upon a time, sell them. We had lots of people interested in converting them to Tiny Homes. I second everything thus far advised and would add a little more:
- Don't let the portable buildings manufacturer insulate the floor. They usually use double bubble foil insulation, which is almost worthless.
- If you get one with shingles, make sure they're architectural shingles over felt paper.
- As has been mentioned, properly insulating the roof - whether gambrel or gable - is a pain.
Not to knock your idea, but - here's a dirty secret - a lot of would-be customers wound up going with a local contractor to build a home "shell." Portable building manufacturers typically have very limited ability to customize the siding, plumbing or electrical packages. The contractor often wound up being cheaper and better able to build a shell from scratch.