I have a pure Swift project that is published on the App Store and has revenue. I built it on a 2015 2.2 Ghz i7 MacBook Pro, but I have an 2017 MacBook 1.4 Ghz i7 with 16 GB of memory that builds and runs the project just fine. A little slower builds, but not major. I am currently running MacOS Catalina beta as well.
If you are going to rely on emulators though, you better get 16 GB RAM. Emulators eat up memory. Personally, I have found iOS emulators to run poorly on macOS on any computer. The MacBook seems especially poor at running the emulators. Although it's not insignificant that it's all beta software. I tried running my app in an iPhone XS Max emulator and it hardly ran. I have noticed emulators have gotten progressively worse over time. I would currently choose iPhone SE for testing in the emulators. You are much better off running apps on an iOS device.
