well it's as straightforward as you listed.
What you have to do is figure out what sound libraries you want to use and how much you want to spend.
You can do it for free with soundfont or sfz or sounds you find on the web using free sampler vsti's like the one at tx16wx.com
Kontakt in Native Instrument's Komplete package is the big boy but it'll run you $500.
And if you're into orchestrations there are various orchestra packages.
You have to make some decisions.