Well there a number of options if you want to notate scores and then either play them back and record directly from the sounds of the program (or any other sound library you may have loaded) or create a MIDI file from the notation program and load that up in your DAW and then load samples and tweak MIDI using your DAW's functions.
Finale and Sibelius are the two flagship products and if you need flexibility of notation, no other programs come close, but these are both expensive and have big learning curves. When I want to notate for performers I usually use Finale. But I have found for quick entry and composition that Notion5 (recently bought by PreSonus so go there to look at it) is easier to use and while it in no way can notate like the other two programs, its notation capabilities are fine for most composing needs. Plus it costs a fraction of Finale or Sibelius. I also happen to like the in-house recorded sample libraries that come with Notion5 better than the Garritan libraries that come with the other two (Notion comes with a default library that you can add to at a reasonable cost). And I have never had a case where Notion crapped out on me for using too many voices. I can work on a huge orchestral score without issues (as long as I don't try and load huge external sound libraries).
There are other options to like Noteworthy Composer a lot of people seem to like. But Notion has an iPad version as well that might be useful (but its too limiting to use quickly for me - but you can transfer files back and forth between the iPad and computer version). So if you have an iPAD you can get Notion for that and try it out to see if you like it at a very reasonable cost (and you can store stuff to Dropbox, etc).
Notion will also let you record from a MIDI source like the other programs and has a quite flexible playback engine that lets you alter tempo easily during playback by tapping a key at the desired tempo, etc. For this piece, I created the soundfile from Notion (where I loaded another string library on top of the Notion library for added realism) and then loaded it into Cubase where I could easily record the theremin and mix the piece. When you deal with strings, you always want to load at least two sample libraries to get a more realistic sound. Then I always run the final .WAV output through a Mastering program (I use Wavelab) where I can adjust/normalize sound file output levels, edit out beginning and ending noise, add fadeouts and add any other mastering plugins I might want - like analog tape simulation to smooth out the sound - and also create the .mp3 file I'll load to Soundcloud.
IMSLP (also known as Petrucci Music Library - www.imslp.org), is the worlds largest repository of public domain and common copyright scores - and also now performances. You can find the score and usually arrangements of virtually any piece by any composer before 1900 there. Living composers (hey people like me) also can post their compositions if they are willing to have them listed as "Common Copyright (CC)". There are various forms of this but basically it boils down to either letting anyone use your music for any purpose under one CC type, or letting anyone copy and perform your music as long as they are not trying to make a profit from it under another CC type. (So don't post anything there you think is going to be of true popular value). And be careful of arrangements of public domain works as the arrangement itself is the copyright of the author of the arrangement.
You do have to be somewhat careful downloading things from IMSLP as they have a lot of music listed as public domain in some countries (like Canada where copyright law is much more liberal than here in the US or the EU - why IMSLP runs out of Canada). So a piece by Strauss might be totally fine in the US, but might still be under copyright in the EU, etc. In general, if you are in the US and the work was written after 1923 - its illegal to copy it or play it without permission. But things are not really quite that cut and dry (i.e. if they were written between 1923..1977 and had no copyright notice). It's a huge quagmire. Best not to perform any anything publically written after 1923 without permission to be safe in the US. EU has a different set of rules.
