I use a 7200rpm 3.5" HD because they are cheaper and faster. There is a huge thread that goes back and forth between "Use 2.5"", "No, use 3.5"", "No use 2.5"", "No, use...", and then the same thing with vertical vs. horizontal mounting. So basically nobody knows. Obviously laptop HD are more resilliant, but the chance of them failing is about equal in my experience. Laptop use less power, but are slower and more expensive. Desktops are power hungry, but cost effective and fast.
As for the sound system, basically behind your stock HU, should be either wires to your speakers, or wires to your stock amp (if there is one). That's what I am unsure of. Basically, the signal that comes from your computer is very low. You need to amplify it. If you have no stock amp, then you must purchase one. You can get a real cheap like 200w amp, and it will work. Make sure it is 4channel if you want front/rear and right/left. That will be the only thing you need to add. My amp cost me $35 shipped from eBay. It's not going to win any sound competitions, but it sounds beautiful to me! If you do have a stock amp, then just direct wire the PC to the sound wires going to the amp.