The 2018 Mini was released in October of that year, so I'm afraid you've got the 2014 model.
With a mechanical hard drive, 7-year old CPU, and under-powered graphics, you are likely to find the experience somewhat underwhelming, which may unfairly put you off returning to Mac.
There can be very long gaps between hardware revisions -- Apple continued to sell the 2012 MacBook Pro until late 2016. So it's always better to check exactly what 'vintage' it is, and keep an eye out on the Apple news websites about the likely date of new releases.
Your Mini can handle 2 external displays, either 1 HDMI and 1 via Thunderbolt, or both via Thunderbolt. Each display can be up to 2560 x 1600.
You need to consider not just the physical size of the monitor, but how many pixels it has, and thus the pixel density. Hi-density monitors work well on macOS; low-density monitors can look a bit blurry. If you can find a 24 or 27" 2560 x 1600 display, that would be good. Then have a smaller secondary display, both physically and in pixels.
To maximize your existing investment, you might consider replacing the internal hard drive with an SSD - though the ifixit tear-down guide shows it as 'Moderately difficult'. Alternatively, you could run the OS off a fast new external SSD drive. Replacing the RAM with 16GB should be cheap and is much easier.
The 2014 model was a cheap entry model, and will still be good for a few years of light work - email, web, Word/Pages, etc. But don't expect to render 4K video or model 3D protein structures. ;-)