Hi @Spy46,
You can absolutely do it with Unreal (and Unity). Since you mentioned you are a designer, I think Blueprints (visual coding) will be easier to learn than C#. If this is the case then I would recommend Unreal over Unity. I would also vouch for the Unreal community being the most helpful community I have ever found for any software! So while you are learning feel free to post in the forums with specific questions as you learn.
Like some posters have said, creating an open world RPG is a monumental task for a single developer, but it CAN be done with a lot of time. I would probably set my aim lower for my first project, but either way its going to be a learning experience.