I have to counterpoint some of what has been said above.
With tools that are available, it is not imposable to create a large scale world in a relatively short amount of time.
You can use procedural tools to create the map, or you could jump in with a large array of applications for building a world by hand ( use Google to see whats available for 3d map makers ) .
As far as a game engine that is capable of running large scale worlds, you have many choices available ( use Google ) .
The longest investment you have is writing a synopsis of the world's history, the main story line, and all the side quests. The hardest part will be implementing all of this into your 'world' and making sure it all makes sense.
The 3D models you can attempt to make yourself with one of the hundreds of modeling tools available, or you could hire some one to make them for you, or you could just buy asset packs and hope you do not have tweek the models too much to fin into your world.