Rob,
It’s only a matter of time before one of the existing models (for FS2004) are upgraded to take advantage of FS-X. It would then be just a matter of persuading someone to do a repaint (a relatively simple matter) This would probably be a better solution than getting someone to design a model from scratch.
Off the top of my head both Aeroplane Heaven/Just Flight and Sky Unlimited do excellent Spit V s for FS2004. The Aeroplane Heaven site has a free download of a Seafire (III?) which is similar and gives you an idea of the quality of their work.
Best of luck,
Bruce
Links:
http://www.aeroplaneheaven.com/home.htm
http://www.skyunlimited.net/