Okay, thanks! Would you say the one I’m looking at is somewhat future proof or should I be looking for a newer one?
I would say that this is a good deal for a 27" 5K iMac, if it does have 32GB and a 1TB SSD, assuming it is in good condition. I would verify that storage is, in fact, SSD not HD.
But at 5 years old, it is a trade-off as far as future-proofing. Sometimes Macs get support for up to 10 years (are capable of running the latest OS), but more like about 8 years is more common. But that can be tricky to predict.
Apple has a history of making substantial changes every so often that leaves a chunk of older Macs behind...unsupported. One of the last big jumps was switching file systems from the venerable HFS+ to APFS.
At some point, when you cannot update, then the clock is ticking. After a Mac is a couple OS versions behind (about 2-4 years) it is not unusual that one can no longer run the latest browsers....and then you can see issues with web: pages not loading or security requirement problems.
This is hard to predict. I just gave away an old 2008 iMac (Core 2 Duo) that could only run OS 10.11 (4 versions old), and can no longer run alot of current software, but will still run the latest FireFox....so still useful for basic stuff with older software. So in this example, at 12ish years old, still running well, but at reduced capability with regard to software that can be run, and maybe 1-3 more years of use, depending on browser support for the old OS.
Short version for a 2015 Mac: I would guess about anywhere from 4-8 years of good usability, with diminished functionality after that, with regard to the ability to run current software.
I tend to think of the computer cost as the cost to purchase divided by the years of use.
- Used for $800, and you get 7 years out of it: $115 per year (assuming the value is $0 after 7 years).
- New for $2500, and sold @ 5 years: $340 per year (assuming the value is $800 after 5 years)
So used is much cheaper...at the trade-off of lower performance, and a higher risk of some hardware problem or failure. All solid state really helps, as HDs more than about 5 years old is the biggest single failure risk. For general computing use, enough RAM and SSD go a long way to future proof.
Last thing (this is all speculation): Apple is likely to move to their own ARM CPUs starting in the next year or two. When they do, the clock might start ticking on all older Macs (all using Intel X86 CPUs) with regard to OS update support. The general notion is when the switch happens, we might expect about 5 years of support for old (non-ARM) hardware. But only a guess on when ARM hits, and how long X86 support lasts. Some folks think it will be longer than 5 years with a phased multi-year multi-year migration, with high-performance Macs taking much longer to make the CPU switch. If so, lack of X86 support becomes a moot point: you would likely be ready for new Mac (more than 8 years from now) before support stops.