I don't think you're getting that great of a deal on the Expedition. I bought a new Ford recently and we paid right around invoice plus there were incentives already and this was an '04. It was an Escape though, not an Expedition. I think the best deals on Ford SUVs are on Explorers, but I can't offer too much insight because I didn't shop for anything that big or expensive.
Also, I don't think this is a great place to ask this question though because people will just tell you the cars that they like or to buy used. Yes cars depreciate and some do it very rapidly but don't let anyone discourage you from getting what you want. It's your money. Not everyone wants a $9000 automobile. I'd suggest driving everything that meets your requirements and then going with your instincts.