As the above poster said, I would start with pencil and paper. Then I would probably move to some sort of CAD software. Although you could possibly do it in PS or AI, CAD is going to be the best to work in. It will be in 3D and give you the best options for output when you need to print drafts or make a mock-up with a 3D printer. I believe things like that need to be made using vector programs, so you might as well forget PS.
I think all he wants is to create a business plan if you will, so I don't think 3D is required.
I imagine Squilly was envisaging something like what I attached.
No, not a business plan. Schematics for the product. Like this: http://www.frankdoor.com/products/images/e_swing_bp.jpg