$950 sounds like a great price and those are super useful machines. I have a 2016 iMac 5K and like it a lot. A couple things to think about or check:
Some of those 1TB Fusion Drives are very very skimpy with the SSD part, so check that out if you can. (Meaning, the fusion drive is composed of a plain old spinning hard drive paired with an SSD. On some of them, the SSD is only like 32GB so it's constantly getting data written to it which causes a lot of wear). You can check this if you go to the System Information app and look under (I think) Storage. It should break it down there as to how big the SSD and HDD parts of the fusion drive are.
The other thing to check for, if you can, is image retention -- images on screen "burning in" after a short amount of time. Mine got that way after about 2 years and it got really bad -- like, after 10 minutes, no joke, I could switch to a dark background on screen and see ghosted versions of my Illustrator control palattes that I'd been using a few minutes before. I read about other people on here having the same issue -- not sure if it was just that specific year of iMac or what. Fortunately it was under AppleCare and I was able to get the panel replaced. If you have access to the machine, run this test: https://marco.org/rmbp-irtest.html and make sure because it's a really ugly thing to see.