Unreal isn't that complicated especially if you're using blueprints and not creating everything from scratch with C++
It also depends on what you're going for... whether it'll be First person, or thrid or something else completely. It'll vary.
I think before even attempting to create a testing map/ prototype using any game engine. That the main concept and game idea is generate first, before proceeding. It's good that you've chosen Unreal that's a start, now all you need to do is get that concept together and then BAM... You have your prototype of course, with the help of anyone involved