You will need a DAW as others have mentioned. I am going to recommend Logic or Pro Tools as these are both industry standards. Logic is Mac only and Pro Tools is mostly Mac only with the exception of a few PC brands.
I will have to elaborate a little on this.
It's not that I disagree with what you're saying: Pro Tools (mainly) and Logic (used to be) are the industry standards. That's a fact for me.
But I would like to ad that there are many professionals, composers, orchestrators, etc, who do their job with other software and computers...
Both PC and Mac have Intel based motherboards now, and the one thing that makes (quite) a difference is the fact that Macs remain with a rather closed architecture, so flaws are much more difficult to be found, whereas PCs are open to anything, which means many more choices, cheaper material, but also more prone to errors in building!
As far as software goes, I, personally (along with other people I know, including Hans Zimmer
) use Cubase. It's what I started with and what I'm used to.
Various experiments with the same audio
files yield the same results, in terms of export (in the same computer), so I doubt there's an issue of quality here. It's rather an issue of what you're used at, what others use (thus the issue of "Industry Standard" is HUGELY important if you're a pro), and what your budget can allow.
Same goes for Finale and Sibelius. I find them almost equal in terms of practicality, albeit sibelius a bit better. I also find them almost equal in terms of results, albeit Finale a bit better. They are both great software, can do pretty much anything, but as far as my eyes are used, I prefer Finale (I started off with Finale, which also means that I'm used to that).
I will repeat, that is not to say that Pro Tools and Logic are not brilliant (And especially Logic dead cheap as well), or that they are not the industry standards. Just that they are not the only ones available... that's all!
Hope you don't mind Charles!