Visit www.cities21.org and talk with Steve Raney. He is an expert in this field.

Has there been any real world data on "sustainability" ?
I know of NONE

How can there be sustainability if you import your food and energy ?

Has there been any data on food sustainability ?

I have been looking for 30 years and not found any.

Can I feed myself and have extra to trade for taxes etc. ??

Trading your town's widgets for another town's food is perfectly sustainable.

An eventual positive trade balance (we'll denominate it in terms of oil) is highly desirable for sustainability, however.

If you're a retirement community or living off welfare, bad things could happen if those checks stop coming.