Does Xcode require El Capitan to run? I am planning to buy a Mac Mini and would like to know the minimum requirements to run Xcode 7.

I didn't find clear information on this topic.
As you can find here, in the Apple Developers webpage:


  Xcode 7 requires a Mac running OS X version 10.10.4 or later. It
  includes SDKs for watchOS 2.0, iOS 9, and OS X version 10.11.


As you see in the attached pic, Yosemite can still be valid to run XCode7, if it is updated to the 10.10.4+ version required.