1. I see no difference in the performance as they are basically the same computer.

2. I've had good luck with Newegg as far as return policy for immediate defects but because it's an HP computer, you'll still have basic support from HP no matter where you buy it.

3. Most of the larger towers like the one you've shown should have an extra PCI slot for a third party audio card.

4. There are lots of custom shops and other brand name computer companies which might satisfy your needs. I can only suggest you check around.

Hope this helps.

Grif