Audio Unit, from midi to audio Options

[crrmi]

hi
I want to start working on my mac on my music (I was a PC user). Now I'm choosing the software. From one point of view I write orchestra or string pieces so I would like to buy a good notation software (like sibelius) but on the other hand I would like to produce at the end an audio. Now my question is: if I decide to buy sibelius and I work on it with a virtual instrument as an audio unit (I'm thinking to buy Garritan Personal Orchestra that seems to me a good product) can I create an audio file? I know that sibelius 3 comes with its kontakt silver player but if I would like to use an other virtual instrument as an audio unit can I then obtain an audio unit or I have to use something like logic?
Another question, is there someone who can tell me how good is the notation interface of logic gold 6? One of my choice is logic gold 6 but I can't download a demo and how it works
thanks
Michael