Smug, Jaded, Enervated.
Join Date: Aug 2012
Location: Terra Australis
No such thing as overkill here!
To answer your questions, firstly, Alphacool radiators are perfectly fine (I have several 180mm rads in my current project, and have used their rads for years without issue), they perform great (They aren't the best out there, but the difference in marginal between the top end rads), they look great, and these days their quality is fine (but be sure to thoroughly flush them regardless)...and for 45mm thickness rads, I don't think there is anything else on the market that competes really.
The XSPC rad should be fine for its purpose, the performance difference between all the super thin rads is marginal at best, but keep in mind, that being 22fpi, it'll require a bit more airflow than the other rads, and probably raise the sound output of your set-up slightly (it'll probably be negligible though), you could just go with an Alphacool ST30 480mm rad instead, they are only 10mm thicker than the XSPC radiator, and far less dense in fins per inch.
And I would recommend going with two D5's, so that if one dies, you have redundancy in the loop.
Otherwise, it all sounds like it will be a pretty solid loop, and impressive system.