Jeremy,
I have used RSpec in the past (1.2), but I tend to fall back to Minitest because it's always there and I have more experience with it. If you're more comfortable with RSpec, though, I'm certainly open to using it. For an API, consistency and meaningful responses (even to errors) are critical. Because of that, testing should focus not only on the "happy path", but also recognizing that garbage can and will be sent to the API and that it must be handled gracefully, quickly, and with useful information for the end-user.
I started using Ruby when Rails 0.12 came out in 2005, and I've followed the evolution of REST along with Rails (including the not-so-good parts of it). You specified Ruby and not Rails, but I'm curious if you have considered Rails::API, or if you'd rather stick with pure Ruby. Most of my ruby apps have been internal or proprietary, so there isn't much I can point you to on the web. I have worked with many REST API's, though, and am currently integrating Stripe into an app for my own use. Speaking as a developer, their API is far better than others I have used.
I am available immediately and can guarantee you 30 hours a week - more as my workload permits.
I realize that my rate is higher than the project budget, but I am a firm believer in the idea that you get what you pay for. I value solid architecture, maintainability and clear communication with my clients, and I look forward to discussing this project further with you.
Thanks,
Jon