Don't let the perfect get in the way of the good.
Even if it can be done... establishing a specialized city or suburb, or even purchasing an island, some practice ahead of time would be in order. It would take years to achieve such goals, so you might as well practice in the meantime.
Shortly after I joined AFF, I started a thread titled "Neighborhood Enclaves", in which I put forward the idea of simply picking a spot and everyone interested trying to get there. For example, look for an apartment complex that is under construction, find out the earliest opportunity to sign a lease, and then a bunch of aspies all show up and rent a considerable chunk of that apartment complex. If the idea doesn't work, then it would simply fade away in attrition, with most moving out for whatever reason over the next few years. If the idea does work, then it would attract more to the neighborhood and eventually influence the local economy and policy.
I've imagined what it would be like if there was a private company made up of autistics. People who work there would probably want to live nearby, thus turning the surrounding area into its "company town". That would influence what types of stores and services set up shop nearby. Something like that may already be happening. You might keep an eye on Walgreen's. A while back, they started hiring autistics to work in one of their distribution centers, and it attracted families to move there from hundreds of miles away so they can work there.
(I haven't watched this video, but it looks like it's about that place:
http://www.youtube.com/watch?v=B2akb4v2cUQ )
As for keeping certain people out, that can be done for buildings and private property that you control, but not for public roads and rights-of-way that are maintained by government. Like anywhere, there will be occasional crime and trouble. In that case, do like you'd do anywhere and call the cops.
As for curebies, how exactly do you plan to screen everyone who enters for curebieism? You'd just have to tolerate them, as long as they respect you even if they don't agree with you. Would you keep an autistic out just because his/her parents are curebies? You'd have a better chance of influencing them away from curebie beliefs if there aren't ugly scenes of them being thrown out, and if they can see the successes of such a community firsthand.