okay.
until now: the installer is 50MB that downloads 7GB, so atm waiting...
source is available from github, with some separated 3rd party dependencies like Recast/PhysX/OpenAL/OGG etc., just requires to register your github name at your unreal account page.
hardware and software requirements are more precise in a docs page: https://docs.unrealengine.com/latest/INT/GettingStarted/RecommendedSpecifications/index.html