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The external PSU is actually good, because it is easy to replace if it dies, as it is generally the weak link in electronics. Just buy a new one with the same output and roughly the same power, and you can continue to enjoy your device.
I wouldn't use this NUC for gaming. I'd use it to drive my main working environment, that is a main 28" 4k display (running at 60Hz) and a side 23" 1080p display. The only app running on this machine would be Chrome, and terminals connecting to my current machines, where the bulk of my actual work would happen. Then I could move the full-ATX tower away from me.
Hackintosh are notoriously hard to install Intel HD graphics, and Yosemite killed support for third-party SSD (at least, the usage of TRIM), but with enough patience, that should be workable. Honestly, that's something I'd try should I get one of these NUC.