Hi,
Do I need Unreal installed in a Mac computer in order to build the app or Can I build it using a windows pc?
Thanks.
You need a Mac to be able to publish to iOS devices. You can still use a Windows pc for your development and just remote publish on the Mac - but OSX is required to deploy / publish for Apple devices.
EDIT: To clarify - you can build Blueprint Only projects on Windows - if you need custom C++ you need a Mac but what I said stands - you can remotely publish.
See: https://docs.unrealengine.com/latest/INT/Platforms/iOS/QuickStart/2/index.html