Mac mini 2014 4k + 2k Monitor Configuration I'm interested in getting a Mac mini Late-2014 (specifically A1347, Macmini7,1, MGEN2LL/A), but I'm concerned by the multi-monitor specifications published by Apple.
The specs state:-

Video Support

Support for up to two displays at 2560 by 1600 pixels, both at millions of colors
Thunderbolt digital video output

Native Mini DisplayPort output
DVI output using Mini DisplayPort to DVI Adapter (sold separately)
VGA output using Mini DisplayPort to VGA Adapter (sold separately)
Dual-link DVI output using Mini DisplayPort to Dual-Link DVI Adapter (sold separately)


HDMI video output

Support for 1080p resolution at up to 60Hz
Support for 3840-by-2160 resolution at 30Hz
Support for 4096-by-2160 resolution at 24Hz


DVI output using HDMI to DVI Adapter (sold separately)


The thing that concerns me is up to two displays at 2560 by 1600 pixels -  the 4K resolution alone exceeds the pixel count of 2 2160x1600 monitors, which suggests to me that it might not support a second monitor at all if a 4K is connected. If the 4K already suffers a refresh rate hit, I'm concerned that a second monitor will either push it into slide-show territory, or not work at all. Obviously resolutions can be reduced, but that's a deal-breaker for me.
So, would 2014 Mac minis be able to drive both a 4K (3840x2160) monitor and a 2K (1920x1080) monitor simulataneously? Additionally, would the 4K monitor maintain 30Hz in this configuration?
It seems, from the spec you've listed, that it should support one 4K display via HDMI at 30Hz, and another display via Thunderbolt.
However, everymac.com does not list the HDMI options beyond 1080p at 60 Hz.
For a 2014 MacBook Pro with the same Intel Iris 5100, everymac does say:
This model supports a simultaneous maximum resolution up to 2560x1600 on two external displays via Thunderbolt. Alternately, it can support a single display up to 2560x1600 via Thunderbolt and a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.
... which tallies with the similar spec from Apple for the Mini.
I'd also draw your attention to the fact that even the 2.6 GHz i5 CPU is only a dual-core, and was considered quite 'light-purpose' at the time.