Owner of a 2012 Mini Server, more in my offices, all have been mated to a high-res display. They are not workstations, rather part of video ingest workflows that run alongside workstations, mated to a second DP input port. Agreed with Miat that you're not going to get "retina" quality due to the GPU limitations in these Minis. What I will recommend is get the highest-pitch display you can afford with an eye to using the display with another computer in the future, offering what I have used a Mini with in the past and present. You've got maxed-out RAM, which boosts the available VRAM.
The displays I recommend with a Mini is the Dell P2415Q, with IMO one of the tightest pixel pitches available in a modern display under $1k - we have several $2k+ Eizo displays to compare it to. A decent second option is the 27" display trio of BenQ BL2711U/Dell P2715Q/Dell U2718Q but I'll only recommend the first two in this group as all of the U2715Q displays we've tested had a nasty green hue and there's plenty of smack about this display with plenty of bad reviews; the pixel pitch of the BenQ and Dell is the same as BenQ's AOC division makes the panels.
Next, get a decent, DP 1.2 compliant mDP>DP cable from StarTech, Belkin, or Accell and look for the fastest thoughput per channel (5.4 GHz per lane). You didn't offer which OS you're using, but you'll need DP 1.2/1.2a at a minimum as the Dell and BenQ displays will require DP 1.2 hardware to work with Sierra/High Sierra. Ditch the DP cabling that ships with the displays, only our Eizos shipped with compliant cables - I know better now.
Don't use SwitchResX - you won't need it. I've used SRX, with a compliant DP cable SRX is not necessary. I don't use HDMI with my Minis. I liked SRX when I didn't know any better while using the non-compliant "stock" DP cables.
Run the display at it's "native" resolution. Scaling the display's resolution will just choke the Mini's GPU, just as it does any retina-capable Mac at any resolution other than "native" and 4k (due in large part to pixel doubling and insufficient throughput with DP 1.2 hardware - I've got an iMP attached to a 4k display and see a bit of "choking" when pixel doubling so that I work with either "native" or 4k resolutions on either of my attached displays...).