We too are just starting our search to buy land and then hire a custom home builder. So far we have learned that sometimes only that city's workers can install things like the sewer line and for example the cost would be 1,000 a ft. And this city also requires a 100ft frontage so we were looking at 100,000 just to install the sewer line from the street.

What you want to do actually costs more than a plain boxy little conventionally built home. Metal buildings aren't designed to achieve insulation levels that residential builds are required to have. By the time you fir it out to do that, you've spent more on both labor and materials. Same thing with the windows that don't meet residential codes. By the time you create the penetrations in the façade and put in residentially approved windows, you will be spending more than a simple, small, well designed, conventionally built home.