I would like to buy a second-hand iPhone 5C and use it for testing purposes. Aside from hardware specs, are there any limitations specific to 5C that I should be aware of? Also, given that I want to buy it second-hand, what should I be looking for to ensure the phone can be used for development?
Short Answer : YES.
In depth : I am a developer and I have all devices, and most models concerning iOS. The thing about developing, as you will learn, with Apple is that they have strict guidelines, guidelines with pros and cons. These pros in your instance work in your favor. Apple requires all apps to be cross-platform compatible, even if the app is not specifically designed for that device. For example, they require all iPhone apps to be compatible with iPads, even if your not developing for iPad specifically, the app just must simply run as advertised in 1x or 2x mode as fluidly as it does for the device you specifically design for. The iPhone 5C is just as good as any other device. Furthermore, you can utilize the built in testing simulator, that has all the devices you may not own. You will have no issues, developing with the 5c, provided who you buy it from sells you a working device.