My first question before anything else:
Are you absolutely confident in your skills at case modding, or willing to learn and possibly make a few mistakes drilling holes and cutting? Because that is what your proposed build is going to require. If not, stop now and rethink your parts. If so, carry on.
As for the other questions I can answer:
Tubing: I prefer either 1/2" ID 3/4" OD, or 3/8" ID 5/8" OD. The thicker walls help prevent kinking when bending the tube. There is no discernible performance difference between the two sizes. The 3/8" ID will be easier to bend in tight spaces, the 1/2" ID will look better if you're fond of bigger tubing. I use Primochill LRT tubing, although some people will swear by Feser and Tygon.
That bay reservoir should be used with two 450 pumps, although it is possible to use only one. Your single loop really only requires one pump, and I would highly suggest looking at the XSPC D5 bay reservoir. The Koolance PMP450 and Swiftech MCP655 are all the same exact pumps, and they're a rebrand of the Laing D5.
I wouldn't recommend EK because of the crap they pulled last year with their nickel plating process, and would instead recommend Heatkiller and Aquacomputer for the GPU blocks, and the Swiftech Apogee HD, XSPC Raystorm, or the new kid on the block, DT 5Noz.
As for barbs vs compression, both work equally well. The main question is, is aesthetics important to you, and how much are you willing to pay for it? Because the cost of compression fittings can easily be 5 to 7 times the cost of barbs, and with barbs for your setup coming out to be ~$20-40, compressions can come out to be $100-200.