Sibelius is more about music notation. I would get a DAW (Logic, Pro Tools, Reason, Live, Sonar, FL Studio, etc) and begin working with 3rd party libraries that have a better sound. Not sure of your budget but ProjectSAM's stuff is right up your ally.
Check out this demo using the library:
Also Spitfire is a good collection of libraries that can make awesome orchestral mock ups as well. It may not be possible with this particular project but it would be a great idea to put these items on your list for possible upgrades in the future.