Your motherboard will come with several sata cables, no more is needed.
Some thoughts on your build:
1. 14nm skylake runs cool, no exotic cooling is needed.
A simple noctua NH-U12s will do the job.
See my liquid cooling rant at the end.
2. Seagate is not the most reliable of drives, look to Western digital.
Here is a guide to your options:
https://www.pugetsystems.com/labs/articles/Understanding-the-WD-Rainbow-674/
3. I am OK with a conventional 2.5" ssd. I changed to a Samsung 950 pro m.2 pcie device and saw little difference except for virus scans.
Most of the performance comes from low random access speeds at low queue depths and that is largely unchanged.
Optane devices are coming and that will change everything.
My canned rant on liquid cooling:
------------------------start of rant-------------------
You buy a liquid cooler to be able to extract an extra multiplier or two out of your OC.
How much do you really need?
I do not much like all in one liquid coolers when a good air cooler like a Noctua or phanteks can do the job just as well.
A liquid cooler will be expensive, noisy, less reliable, and will not cool any better
in a well ventilated case.
Liquid cooling is really air cooling, it just puts the heat exchange in a different place.
The orientation of the radiator will cause a problem.
If you orient it to take in cool air from the outside, you will cool the cpu better, but the hot air then circulates inside the case heating up the graphics card and motherboard.
If you orient it to exhaust(which I think is better) , then your cpu cooling will be less effective because it uses pre heated case air.
And... I have read too many tales of woe when a liquid cooler leaks.
google "H100 leak"
-----------------------end of rant--------------------------
Your pc will be quieter, more reliable, and will be cooled equally well with a decent air cooler.