Like Legend said. The CPUs come with fans. Very rare that they don't. I haven't had any issues with the included fan.
Mouse, keyboard, monitor, the optical drive, network card and speakers I already had.
Mouse, Keyboard can be gotten pretty cheap 20-40$.
Monitor is going to be somewhat expensive depending on size/resolution. And you could use your TV.
Optical drive maybe 20$.
Network card, difficult to find, but may be able to find one for 20$.
Speakers, 20$ maybe more for better quality. Less if you know where to look. If you're using your TV, not an issue.
As for the OS, that'll probably be the most expensive thing, barring a new monitor.
Keep in mind those are US prices.
Only thing I really need is OS Mouse and Keyboard
I just recently bought a new thirty two inch TV that may be a bit big but later on down the line I will get a monitor in the twenty inches area they dont usually cost as much as a TV
Also I would have to get a headset now lol after using the ps gold headsets i just cant go back to using the speakers on my TV