I am leaning towards a pre-built shed and adapting to a coop.
With a shed would rot be an issue from sitting on the ground/gravel?
Nothing wrong with going prebuilt (I'm not too handy, hubby is busy and would cut his own arm off - had a professional do the build and install and he had it up in 2 hours) but you'll need to make some changes to it, like adding ventilation, roosts, nests, etc.
If you buy a shed with a floor the entire unit should be sitting on pressure treated skids and/or concrete blocks. If you're going with a dirt floor then I imagine anything in direct contact with the ground should be pressure treated and marked for in ground usage (not sure on that).