Yes, you can get a rather sufficient one for free at flightgear.org for free, you can then download from a variety of hundreds of aircraft made by other users.
A flight simulator has many different aspects for FAA approval. Take a look at aeronautical systems engineering, leading Avionics Simulation design flight simulation,Flight Training Systems. Building...
You can't, the coding on the planes is different. Most of the planes you find in Microsoft Flight Simulator 2004 are available in Microsoft Flight Simulator X.
There is no need to buy Microsoft Flight Simulator X: Deluxe Edition as Microsoft Flight Simulator X: Acceleration contains everything from Microsoft Flight Simulator X: Deluxe Edition and more.