I can't see anything there that needs a G3/8" to G1/4" adapter, as far as I know only thermochill use this thread.
Either D5 or DDC are fine, I'd probably buy the D5. EK sell an aftermarket top for either which nominally improves performance but more importantly lets you use normal barbs with it, I don't trust the built in plastic ones.
Which radiator isn't really the question, it's more which fans on which radiator. It's important to match them. The bigger the radiator, the slower the fans can turn. Alternatively the lower temperatures will be if you don't turn the fans down. Thin radiators (30mm thick) will need high static pressure fans at high rpm, thick radiators can remain efficient with quieter fans involved.
Finally I don't like bay reservoirs. I've not used one, but the design looks like it's the most likely part of the loop to fail and leak everywhere. No doubt this is paranoia on my part, but when a T line achieves much the same result I'm reluctant to use a reservoir.