Also, do you know why the 360 kit is cheaper than the 280? is there something I'm missing?
If you print off the spec sheet of each and compare, the main difference is that the 280kit is using 2 x 140 fans whereas the 360 uses 3 x 120 fans. The 280 is very useful if you do not have the space etc.
Why is it more expensive - simply, I think, supply and demand - if I interpret correctly ,as there are only a few 280 kits about a premium is not too surprising.
(There may be a few reviews about - so if you wish to be sure see if you can find and check for yourself)
5 litre of de-ionised. Inexpensive and keeps. All one does is pour it through the radiators etc to make sure they are clean before setting up and filling with the actual fluid you intend to use. i.e. it is just to clean through.
I would think that one litre of Mayhems fluid would be enough - it does depend on how long a run , how many rads , blocks etc BUT I found that 1litre was more than enough to run a loop in an 800D case i.e. Pump>Top (triple) rad > cpu>gpu>pump.
I did go for the concentrate (which if memory serves makes up to 2 litre) and bought 2 litre of Mayhems pure water but I overdid it - one litre was more than enough and I would expect it to be enough for your cpu alone loop (although until you build who knows

)
If you decided to go for either kit - and I would go for the 360 kit if I were you (better bang for buck- provided it will fit

)- then assuming that xspc have not changed their approach then the kit will contain a very useful guide!
Extra's needed apart from the fluids - I would change the fans but you could leave that till later

- I was not keen on the plastic hose clips that the kit came with (but they did work and no failures!) so I might treat myself to some metal hose clamps (if I could find them I would link you

) Def need a sharp knife/scissors (to cut the hose) and some funnels (so that you can get the fluids in without disaster!) - Offhand nothing else comes to mind apart from mentioning the fact that if you want all 3 rad fans to run at same speed then you will have to link them to the same power point .
I did not keep records of cpu temps. to compare before and after when I moved to the 360 kit as I had been using a Corsair H60 so it was cool already - my main reason was to cool the gpu , which it did

I ran the rad fans on PWM and as I was also cooling the gpu it would be unfair to compare before and after anyway BUT the loop certainly kept the heat in check. Running an i2700k @ 4.5 it did not go above 60c even using IBT
I would mention that in my experience moving to full water from a sealed unit for the cpu alone was not worth it - might be if you wanted to try for as high a speed as possible (more volts etc) but only marginal gains otherwise.
I do not regret moving but the cpu was already overclocked and quiet - the gpu however was awfully hot and noisy (with 2 unbearable) and water tamed it.
Sorry if rambling

Hopefully some of this is of use