well pc doesnt realy matter if u play at 100 fps your good higher is better like always
keyboard depenst on your taste what u like an feel comfort in the mouse well also your own feeling most player in csgo have a big mouse mat with low dpi around 450-700 dpi the the most used dpi for csgo but...
mean the i5 6600k with a good cooling bit of overclockern en your fine to go for a few years there a not a lot of games that use a lot op cpu most game that come out are 90% gpu based so a i5 6600k will do fine
i would go for i5 get 16gb of ram go for a msi nvidia gtx 1060/1070 depents on budget and lose 1 ssd its better to have 1 hdd and 1 big ssd then 1hdd and 2ssd u have right know and maybe make hte 550watt psu to 600
that motherboard hase 4 slots for a sata so u can add your hdd ssd and optical without problem jes u need to buy some cable but they const hardly anyting maybe 5$ for 2 extra kabels
if this is your max buged then change the hdd for a ssd-hdd comb mean change the 2tb harddrive for a ssd and harddrive use the ssd for fast boottime but it is a desent build if this is all the money u can spend