When I no longer can run the latest OSX on it, or iTunes. With my current computer (2006 Mac Pro) I no longer can upgrade my iPhone without giving up being able to sync it and my music to iTunes. Also I'd like to be able to run Civilization VII and perhaps even Civ VIII eventually.
Not sure what you mean here. It's a used iMac so I don't have any other option than the 1TB fusion drive it comes with. However, as it is the 2014 model, it should have the 128 Gb SSD portion, and that's a good thing I guess.
I'm sorry, you are correct I thought I read 2015.
There is no future proofing Macs when it comes to official OS releases and proprietary software.
Their updates are arbitrary and more based on particular year minimum specs. They tend to be more related to the specs of other things with the machine like Bluetooth, USB versions and things like that. Older devices start losing newer features because of that.
That's why you typically don't see their new OS's make exceptions for older machines (e.g. 2015 iMac or later; except a 2014 iMac with xxx GPU or something silly).
So in that regard I would get the latest model. And get specs you'd find useful for the task you are performing. Maybe consider specs you'd want to run a different OS toward the end of its life.