I also am using a Mac mini with similar specifications, and it is more than fast enough for Xojo. The monitor I have connected to it is a 24-inch Dell U2412M IPS monitor, and the image quality is incredibly sharp, almost as good as a genuine Apple monitor. Although the 24-inch screen seems adequate, a 27-inch or larger monitor would be better.
One of my Macs is an older 2010 mini. I’m always surprised at just how quick it is. The newer ones are even quicker. As a bonus, if you want to continue using Linux, you can run it from within VirtualBox (free), Parallels (my choice), or VMWare Fusion (also very good). If you ever need a performance boost, I would suggest upgrading RAM to 8GB. Big difference on large photo libraries for example.