Sibelius is an excellent package for orchestral arrangement, especially good when you combine it with Alesis Nanosynth (£110) external midi synth generator.

downside is if you get Sibelius at retail price its £600
so get one from a university or buy it with student discounts

its is good as it can add a human variance to instruments the odd millisecond mistakes that make arangements sound interesting and not digitized