Open an app from an unidentified developer If you try to open an app that is not registered with Apple by an identified developer you get a warning dialog. This doesn’t necessarily mean that something’s wrong with the app. For example, some apps were written before developer ID registration began. However, the app has not been reviewed, and macOS can’t check whether the app has been modified or broken since it was released. A common way to distribute is to take an innocent app and insert harmful code into it, and then redistribute the infected app.
The safest approach is to look for a later version of the app from the Mac App Store or look for an alternative app. To override your security settings and open the app anyway.
XCode is free and includes an emulator for testing your apps, so you could get away without a iOS device, at least 'til you've got over the initial learning curve of Objective-C. There is a $99 a year iOS Developer program but you won't need this until you're ready to put your app into the App Store. If you need a new one, the Mac Mini is the cheapest. If you have the mouse, keyboard, and monitor all you need is the computer itself. Make sure you get enough memory as its not upgradable! Really though, you'll be fine to get a used Mac. In fact, almost any Mac from 2008 or newer can run the latest OS. Put enough memory in them and an SSD and they fly.