Well, I'll skip the orchestra stuff since I know nothing about them. However, all VSTs use the CPUs, memory and hard drives of your computer. That is where you should put your money, esp. the hard drives if you are doing orchestral stuff. The common hard drive configuration is 1 for the OS, a separate drive for recording audio, and if you have a large sample based synth like you'll use you should probably have another drive for that, esp. if it "streams" the audio.
You will need an audio interface, since onboard cards ain't made for recording or midi production. And their reproduction is low quality. there are too many units to choose from and there are plenty of threads. Decide how much you can afford and get one. Focusrite and Roland are common amonst the forum members and seem to work well, but all audio interfaces have the potential to be buggy depending upon your configuration. Just be sure to get one that you can return easy, tho whatever you choose should work. I'd go w/ USB today, since firewire is losing share amonst computers. I'm happy w/ my TC Konnekt 48 and would advise a twin or 6, if you can use FW w/ the computer. Not the greatest latency, but above average ADDA. The Focusrite Forte seems to be entry level price for sound quality these days, although w/ in-the-box productions the conversion is not so important, exept for monitoring.
Sonar works fine for what you want to do, except for the scoring part. Another thing I don't use, but I've been told it is weak. If you plan on scoring, you need a different or get one of the scoring programs and use SONAR to mix, etc.
As far as midi keyboard - most use USB these days, more than have actual midi in/out. Go to a store, if you can, to test the keyboard beds. If you play, it is important to have one that feels right.
Get a well spec'd computer, an interface that works w/ it, and the rest is gravy.
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