not sibelius, the instruments sound like crap and it doesn't do trills or anything agh it's so annoying sometimes!
Yes, it costs a few thousand dollars to get sounds that are "acceptable" -- They sound like a video game, but you can tell which instrument is which. You need a sequencer, keyboard, MIDI hookups, and a synthesizer. The synthesizer is just a box that produces the sounds. Depending on how good those sounds are and how well you know how to use a sequencer and keyboard to recreate the instrument sound you want -- that will determine how close you can come to sounding like the real thing.