The more you pay, the more you get, so to get an optimal advise I need to know your budget.
If you go for the good sounding stuff, a sequencer will do the job. You can try something out of the Sonar line, or Reaper, a cheaper, but nonetheless very flexible and it'll probably cover your needs (for the sounds at least).
For the sounds themself you should buy a decent ROMpler. They range from the budget types (Garritan, EWQL Silver Orchestra), to near-realistic orchestra emulations (EWQL Platinum, Vienna Symphonic Orchestra).
And for the sheet music, you can't do that properly with most sequencers, so Finale Printmusic will do the job. Finale can also be used to get your sounds, but sequencer like Sonar and Reaper offer far more flexibility, resulting in better sounding songs.
I use Sonar 4 Producer Edition with the Vienna Horizon Opus library, but I'm a lucky boy (my uncle used to be a hobbyist composer, until he got to busy with work, and that gave me the oppurtunity to buy his stuff really cheap

). You can't hear an example of my music, since I still suck

, but if you have any further questions, feel free to PM.