Hi Andrew
Apologies in advance, I'm going to be *that guy*... ;-)
For what it’s worth, I’m a huge fan of Linux, Lilypond and
Muse and all the other wonderful *free* tools that we have available.... BUT if I was in any sense interested in working with a
commercial publisher – especially for paid work, I wouldn’t mess about. I’d buy
a copy of Windows, if I didn’t already own one, and then buy a copy of
Sibelius. That’s just what they expect; and quite reasonably, as it's the industry standard!
Also, if you’re expected to give them a well-presented
score rather than just the notes, you’re going to need to do some work, possibly quite a bit, within Sib. Photoscore will give you the music entry at higher speed than manual entry, but
in my experience of Sibelius (mainly the now old version 3), getting a piece to performance quality output requires quite a lot
of manual work to move symbols and so on about. (Though it's possible they've improved this by version 7, and some of my friends that use it say this is the case).
Best of luck :)
Chris