If this is a single project, I would just get started using the engine, going through tutorials, loading and experimenting with models, etc. It's very common to pick too large of a project to start, so you should weigh that up considerably. It almost sounds like you haven't written a game yet, so it would be better to start with a simpler game like asteroids and get a feel for everything like moving objects around, making a menu, making levels, collision detection and interaction, particle engine use, etc, etc. Once you've written a small game, a larger game is just an expanded version of everything, so you will have a much better idea of how to design it and what you are actually capable of completing. Once you move into character type games, you have to probably consider AI and path finding before designing your game, so there is a lot to learn before you need to worry about something like a design document. That's a much more high level type of thing and isn't really necessary unless you are in a team as far as I'm concerned. You can just as easily work off an outline if you are a single developer. A survival horror would probably be done by a team, so you would have to take that into account, probably changing to smaller worlds with other type of game play because modeling large worlds is too much for one person; or finding a team to work with, but teams can be a major headache if they are not commercial.