so no nice clean looking ones, like the one that came with the SD3?
they are open metal cased power supplies?
In my experience normally they are, certainly the affordable ones. The SD3 one is big for a soap-on-a-rope, it's unlikely any other of a similar form you can find is more powerful - maybe someone else has a tip?
Unfortunately this job is not a plug-and-play scenario. There's cable chopping and wiring and mucking around whichever way you look at it! Compared to the metal frame ones, the ATX option avoids having to do anything with the AC side of the supply, which may be a small bonus - everything you have to do is low-voltage.
Yes, murata is a good brand, 12V at 33.3A (400W) is more than enough power. However it's listed at $244, no stock, and the datasheet is marked as 'obsolete' (murata will have released a new equivalent model). It has a bunch of features (load-sharing signals and serial control etc.) you won't use. The one I linked earlier was $60 for 350W (also more than enough), so much better bang-for-buck. I figure when you purchased the E3D you didn't account for the cost of a new supply, so minimum spend seemed an implicit requirement. 
SD3. Mk2b + glass, heated enclosure, GT2 belts, direct drive y shaft, linear bearings, bowden-feed E3D v5 w/ 0.9° stepper
Smoothieboard via Octoprint on RPi
