Now that Mac OS X 10.14 "Mojave" has been released, it's time for me to write once more about how annoying it is that there are no reasonably good systems you can buy to run it on.
As I mentioned before, the majority of Apple's product line is shamefully out of date for the planet's most valuable company. Very few new Macs are worth buying at their prices, and so there's nothing reasonable or even upgradeable to buy if you want to have a Mac but you don't, strictly speaking, want to run it as your main computer.
For someone who needs a Mac, there's a reasonable justification to just buy the computer you need today. Used Macs are an option, probably the best in this case, but there is the challenge of buying something reasonable configurable with the intent to keep it up to date or buying something that comes the way you think you'll want in a few years to have bought it.
It's tough to justify getting a Mac if you just like having them around. For a casual user of Macs, and for someone who has just a few Mac OS X apps, my personal advice is to just stop, unless your needs are truly simple enough you can get away with the old baseline Mac mini. (I think you shouldn't, but you could.)
The used market is probably the best bet to get a machine. A 2012 Mac mini will likely cost almost as much as a 2014 Mac mini does, especially the Quad-core models or the version with the discrete GPU, which never felt like a very big value add for me. The 2012 Mac mini is one of the last "consumer" upgradeable Macs, so it can run 16GB of RAM you can buy at any time and up to two SATA disks.
The MD101LL/A, a long personal favorite whipping boy of mine, primarily for being the distasteful favorite of people who want "performance" and "expandability" without actually caring about either, should run 10.14 well enough, once a solid state disk was added. Basically, any machine from 2012 with an SSD in it or to which one can be added would be "fine."
There's only one or two machines I can categorically recommend against:
The first is the "2010" or "2012" Mac Pro. This machine, with the model identifier of 5,1, is still going for a lot of money because you can expand it internally to have most of the same functionality of the newer 2013 model. I suspect either of these machines would be given a run for its money by some of the newest mainstream hardware if an 8th generation 6-core chip wouldn't do it, a 9th generation 8-core chip almost certainly would. IPC has doubled since there, HyperThreading is still there on i7s and every other component has at least doubled in speed.
Ultimately, just as I said a few weeks ago, I don't know if I agree that there are good Macs to buy right now. The two machines that have been updated to 8th generation chips cost a lot because they also feature Touch Bars. Everything else in Apple's product line is meaningfully outmoded or simply a bad configuration for anybody who wants to do anything at all on a computer.
Ideally, Apple would be announcing new Macs in the next few weeks. There's a holiday season approaching and even though they've missed the big order deadline for academia, schools (at least higher education) do order machines year-round.