You probably would like using a notation software, and not needing many of the functions in Cubase, etc.
Sibelius and Finale are most popular, try the demos.
For orchestral sound you can pay a look on Miroslav Philharmonik or Garritan, which shuold be compatible with the programs mentioned.
You'll need a powerful pc to use lots good sounding software intruments, and maybe you'd benefit of a audio interface with ASIO drivers.