I'm curious as to why you would be so dependent on the stream. Don't earthship designs include rain catchment into a large cistern? Or are you in the high mountains where there isn't much rain?
Also, unless you get lots of help, building an earthship will take quite a while.
I do think you are right about getting a property with a spring on it. It will only help (unless you find yourself in an area that is regularly flooded, which I assume you are going to try to avoid ). Have you heard of helpx.net ? This could be another source for you to find temporary help with your project- you could put up a posting that says "Hi, for two weeks during "x" time period we would love to have 5-10 people out here pounding tires and eating great raw food in Ecuador, thank you!" I imagine there would be travellers contacting you.
I imagine you know about Raw Food World. Are you going to their gathering next month? Check out their newsletters and TV show. Angela and Matt Monarch are in Ecuador. It is legal to buy land there and not so hard to get a residency visa. I would caution you to engage with local people and think through the consequences of everything for your neighbors. If you dam a stream, who is going to have less water downstream?
In the mountains the streams can get very violent, so pay attention to the high water marks and move even higher still, since that country is getting a lot of ex-pats, and therefore development and therefore some deforestation because of clearing for building and farming and firewood. Locals are mostly farmers, too. There is more runoff all the time, and faster. They have a rainy season and a dry season. Like most places.
Also think about the volcanoes and earthquakes. One whole town in Columbia got swallowed up in a few weeks time, little by little. Amazing. It was on a steep hillside. Study geology. Also Spanish.
It seems to me that Ecuador is like Mendocino County in the 70s. I might even end up there myself to stretch the social security check and dodge the pollution. However, the politics will likely change. One biodynamic farmer mentions getting robbed at gunpoint, so security is always a consideration. They may think of it as redistribution of wealth. Often there is resentment when change happens so fast and they can't buy affordable land themselves because the yuppies are gobbling it up. I also feel that it is not right to flee the mess up here and go somewhere else pristine and thereby make it less pristine.
Just some food for thought.
I suggest you go to Matt and Angela's gathering anyway. Many people will be there who are not so extreme. There will be people from all over Ecuador and the world there who can give you many tips. Last year it was a small, unstructured gathering. I have seen many of their Ecuadorian friends on their crazy TV show and it is a loving, helpful bunch. Matt carries everything to extremes, which makes him entertaining to watch, but I don't try to imitate him at all. I actually eat all raw sometimes. Other times I eat more macrobiotic. Other times I eat whatever is growing, or affordable, or served. Food is survival and enjoyment to me. It is not religion or a business.
I wish I could go. Maybe next year. It is because of the people I have seen interviewed on their show as much as the land. Both are very beautiful.
I suggest you go there, look around all over and see where you end up, because you will meet others and learn things you can't find out about without being there. The same is true of what you end up building. That depends on what materials are available and the cost. Look for blogs of people there.