Actually, the standard is FINALE--very widely used, but not really well liked--it is designed to provide every option that you might need for orchestral parts--the price is probably over $600, but you can get a "educators" version for about half that--I would suggest getting in touch with someone connected with a music school to see if you can finesse a copy--

I bailed out on FINALE and use Overture, which is much more friendly and has a very good sequencing capability--it is an orphan program though--developed by OPCODE, then supported by Cakewalk and now gone I don't know where--it wasn't cheap either--

If you use a MAC, there is a free program called Virtual Composer at the Apple site that looks quite good--

Composing software offers amazing capabilities, but the price, in both cash initial cash outlay and learning time, is pretty steep--good luck!