The big issue I see with such a "sustainable ecovillage" is what I call the long term impedance mismatch between the village and the external environment. So while I am pessimistic (and I think you are right to be "worried"), lots of people are trying it. Many are in the US.
In what way could we seek to establish a sustainable ecovillage?
- Ecological sustainability
The village uses only those resources it produces. Production can be primary, secondary, or tertiary.
If the village is to be sustainable at the primary level, it needs about 1.5 hectares per person. The amount can vary according to the natural fertility of the land. Primary production includes crops for food and raw materials (examples could include cotton, flax, timber), animals (perhaps fish, cattle, sheep, goats, silk worms), and timber. Notice that I have excluded minerals; although we might like to use clay and other minerals, they are a finite exhaustible resource.
The secondary level of production may include clothing and housing. We could imagine clothing made from leather, wool, silk, cotton, or linen. We could imagine timber furniture.
The tertiary level is services. We can imagine everyone turning out to help with seasonal activities, providing their services to plant or harvest crops. Other more specialized services could be provided by teachers, midwives, etc.
The issue is that without external inputs, for example steel artifacts, we don't have horse shoes, axes, plows, knives, etc. Perhaps we can make substitutes for some items, but what do we do without glass, and other manufactured items requiring heavy industry? This is a rhetorical question. The answer, of course, is that the village must trade with the outside world.
- Economic sustainability
We have established that the village must trade for articles it cannot produce. (This opens a different question: do those articles come from sustainable sources, or do they use finite resources? Clearly, at some level, they use finite resources.)
In the short term, We might imagine that the village operates a farmers' market, where the surrounding population could trade with the villagers for primary (and also secondary) produce. In the short term, we could imagine that this could achieve an economic balance.
But in the long term we must consider that people age, become unwell, relocate, commit crimes, have children, and all of life's non-ideal issues. We must provide health services, and deal with the lazy, the crazy, the criminal. These services are costly in our current society. It is not clear (to me) how these costs can be sustained by the village, unless it is fully engaged in the wider economy.
Further more, as people age, will they stay? As children age, will they stay? How will the village maintain it's population? This has been the Achilles heel of such attempts, and is an ongoing problem for rural populations. How will we ensure that the future population follows our "rules"? We cannot, because our rules cannot be complete and cater to all possible contingencies.
So there are significant challenges. We have not even mentioned the question of what technological level the village will use. If we use high-tech items such as computers, mobile phones, solar cells, or even lower tech items such as wind generators, tractors and other machinery, we extend the question of what is sustainable? No matter how good our recycling efforts, finite resources become depleted. This, of course, is the world-wide problem that Sustainability.SE serves.
The big issue, it seems to me, is that this is being analyzed as a self-contained entity. It cannot work that way, any more than any other village does. Any village has it's rich, poor, workaholic, lazy, more ideological and less ideological. People move in and out. Land is bought, sold and rented. Some people have cars, trucks, and tractors, others use bikes and draught animals. Kids grow up and move on. Some return. But every village has it's own character. It is made up of the sum of the character of the inhabitants.
My advice is to just do it. Find an existing community or just a few like-minded people, and get started. Add your weight to theirs. Actively promote the sustainable culture. Lead by example. Be inclusive rather than prescriptive.
In Australia many villages have collectively chosen a particular focus, mostly to project a clear identity that can attract like minded people and tourist trade. While these are not Intentional Communities or focused on ecological sustainability, they are economically sustainable; a few of the many examples of where this is working include Nimbin, Milawa, Yarragon, Darnum, and many parts of Tasmania. You'll notice I've used tourism links, that emphasize the character of these places.