What would you recommend spec wise for a 2012 MM, what res / ppt being the sweet spot?
A 2012 Mac mini can't run a 4K display (which is the minimum resolution you need to get the HiDPI modes with their vastly improved text rendering
out of the box) at 60 Hz. The highest refresh rate the hardware can manage at 4K is 41 Hz (using CVT-RB timings) due to a 360 MHz pixel clock limit imposed by the hardware itself, but macOS further limits pixel clock to 270 MHz, which reduces the maximum refresh rate at 4K to 30 Hz (using CVT-RB timings). If you don't mind lower refresh rates (which will make e.g. the mouse cursor behave sluggishly), you can go for a 4K display.
The highest resolutions you can run at 60 Hz given these constraints are 2560×1440 or 2560×1600. But in order to get into pixel density ranges where HiDPI modes "really make sense" then (say, at ≥160 ppi), you'd have to use small (like 13" to 18") displays. There are portable monitors which have 2560
×1440 or 2560
×1600 resolution at that size but they may be too small to use as a desktop monitor. The smallest 2560
×1440 desktop monitors I'm aware of were 23.6" or 23.8" which is ≈123 ppi. But you could get a 2560
×1440 monitor and try a custom 1920
×1080 HiDPI mode. This will give you the same real estate as a 1920
×1080 monitor but text will look better than on a real 1920
×1080 monitor. My own tests using a 27" 2560
×1440 with HiDPI modes have shown that I notice text quality degrading visibly after 2048
×1152 HiDPI (which the 2012 Mac mini's GPU can't do though), so 1920
×1080 HiDPI might be fine. But it's all subjective so you won't know until you try it yourself.