Answer 1

UDK is a free game engine you can download from Unreal's site. It got a pretty amazing upgrade now as it was rebuilt from zero into the brand new Unreal Engine 4, that's also free to download and easier to learn/use. It's one of the most advanced engines in the world.
UE4 has super nice features, including a repository of pre-made content you can download from its exclusive store (both free and paid) ready to use as a startup/template to or be incorporated as add-on into your own game projects.