Ultimately theres no substitute for grunt, so get the fastest "middle order" processor possible - the fastest ones are always IMHO prohibitively expensive relative to the performance gains made. AMD usually give better performance per buck.

Ignore anything you are told about processor upgradability. What is true now is usually not worth the hassle in 3 years time because everything moves on and upgrading just the processor tends to highlight other system bottlenecks.

You aren't playing games or doing video so almost any graphics card will do, even the basic "on-board" stuff. Just note that some on-board chipsets "steal" main memory to use as video ram, although the amount is not great.

If you are going to record audio you need a low-noise sound card; on-board or budget will not be good enough. I've got a Creative Audigy 1 which I figured was decent enough for my tinkering and is nice and quiet. There are many other alternatives out there.

Of course whatever you buy will be available cheaper in 3 months and considered obsolete in 12. Ho hum.