Except that the "modern world" has a knowledge base and a technological infrastructure which allows you to do things that would be impossible from the Amish starting point.
Well, otherwise it wouldn't be possible to "upgrade" an Amish community! I agree that the technological infrastructure is something off a cheat; it would need rethinking. (How do the Amish get their asphalt anyway?)
My point is that the technological infrastructure is not a cheat, it's precisely what allows you more control of the way to get there from here, and it may be what makes it possible to begin with.
I mean, suppose the Amish wanted to build a wind turbine. Are the turbine blades going to be made or wood, or wrought by an ironmonger? Where do you get advanced materials from? Those whom the Gods wish to destroy They first make mad. -- Euripides
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?
According to professor Nomad of the Netherlands, never
There, the waste material is brought to normal air pressures and a yeasting reaction starts - producing methane and CO2 which can be used to produce electricity. The first estimates predict that about 10 percent of the houses (so 3 out of a total of 32) can be sufficiently powered this way.
Or are you wondering how large a community has to be before it can build the infrastructure necessary for 10% of its energy to come from poop? Those whom the Gods wish to destroy They first make mad. -- Euripides
But also: Is there a point (scale) that the return gets larger than 10 percent? If we have 500 people, do we get a larger return, say 15%, or will it always be 10%? I've learned in Sweden that a follow-up for some 500 houses in Sneek with the same sewage system is practically go, so we might actually get an answer on that question - in a few years.