Hardware requirement for developing iOS app

As of November 2018:

Xcode 10.1 requires Mac OS 10.13.6.1 (macOS High Sierra) that requires iMac 2009, Mac Mini 2010 or newer.

To prevent being obsolete in the near future, one should get a machine that can run 10.14.1 (macOS Mojave) that requires iMC, Mac mini late 2012 or newer.