I have just gone through this exact experience and have had good success with the developer I got through ODesk. If you want to have a look, we just soft-launched on IndieGoGo and are going full-launch on Tuesday.
You can see screen caps of the tool she built both in the video and some stills in the Media Gallery tab at top.
Some advice from my experience:
2) Write up their proposed technology stack and explain why they made the choices they did
3) Propose a workplan, with milestones and estimates
4) Code up a single page from a wireframe, with some basic functionality.
At the rates I was getting on ODesk, it was going to cost only about $750 for the test phase. But it saved me a whole lot more than that.
One guy was fronting a group and didn't follow my instructions. He put in 30 hours (said he'd only charge me for 10) and did a good job on the page. He never turned in the tech or work plans.
Another guy (with ridiculous qualifications), didn't hit the deadline, didn't seem to be working on it. Then he emailed me the night before the deadline to say that he had been sick and could he have an extension. I gave him another couple days and he sent me a file that I couldn't open with a browser. No tech or work plan.
Then I got a 2 page tech plan, a big MSFT Project Gantt chart, and a page of working code from a woman in Sri Lanka, who would email me every couple days with clarifications. Her page looked the worst, but that's because she followed directions and crammed it into 10 hours. And she did her homework and took the time to make sure she understood me.
So my takeaway for you is this: You can do it. Run a process. Put a PREMIUM on communication skills, because you'll be emailing back and forth and miscommunication will cost you a lot. You want someone who doesn't assume, but instead asks questions to make sure she knows what you want.
Things are going great for us and we just hit 35% funding at http://igg.me/at/fiskkit before we've even hard--launched. I'm looking forward to my ODesk Developer taking us all the way to completion of MVP some time in February.
Feel free to ping me with any other questions. Happy to help.