Biggest mistake I made when I bought a new monitor a couple of years back is that I went for a 2K monitor instead of a 4K monitor. You don't get Retina mode in macOS with a 2K monitor, so unless you buy a huge one (which I didn't), your UI is going to be tiny. The way macOS does UI scaling is with virtual pixels (to retain software compatibility) and it can only do so at integer ratios—thus the 4K requirement.