The Mac Mini has good sound. If you want 'really' good sound, you'll need a sound card, but it will have to be external.
Keep reading up on on front ends. Take a look in the software forum for lots of frontends. Road Runner is the most popular free front end for PC. Streetdeck and Centrafuse are two of the pay options that are good. The selection is much slimmer for the Mac.
Hook up your computer and your screen to your power supply so you can get regulated and clean power to them.
Check both the FAQ link in the nav bar at the top of this page and also the FAQ Emporium's 'FAQ to the FAQ's' for way more info. If that's not enough for you, look in the wiki for info on front ends. Try search terms "what is a front end?" for additional info.