On available infrastructure - it's the one I was going to blithely ignore to avoid instant headaches, but you've me cornered now. Like your turbine question, the same is applicable for plugging in televisions, radios, washing machines, etc - you'll need materials, facilities, transport. It relentlessly enlarges the perspective - perhaps to the point it's actually realistic to integrate the community within an equally sustainable society. But that's too much for my bookkeeping skills...

Therefore I thought to simplify: I'd start by ignoring all of the above, and just begin with everything that's presently available. From iron wind turbines, to chemical toilets, just assume you've a giant stockpile of everything at hand, Community SimCity 2000 with unlimited funds. Then, move on to the larger perspective and see how sustainable the community can actually stay.

On scale: Also very valid, and in connection to the above point. Adapted to a simplified community, lifting on the technological advantages of the modern world, I'd use scale for at what point modern techniques can be adopted for a community. Say, greenhouse techniques, or housing. At what point could your community cook on poop? 500 people? 1500?

Anyone interested in programming SimCommunities 2000?