To toggle between monitors?
Not sure what you mean exactly, but you just move your mouse to the edge of the screen and it goes to the next monitor. Using multiple monitors isn't really any different to using 1 monitor. You set 1 of your monitors as primary, which is where new programs open, then you can use hotkeys or drag the window to wherever you want it to be.
W8.1 is the best, but W10 should be releasing soon, which is a free upgrade for W8 users so you can always upgrade when it comes out.
Parts list looks fine, just pick keyboard/mouse you want I guess. There's also higher airflow fans but they will be louder (you can get fans that move about twice as much air).