Apple's refurbished products are better than new ones imo.
If you find what suites your needs in Apple's refurbished store, I would go with that. But, I wouldn't get an iMac with a HDD, and I wouldn't get the 1TB Fusion Drive in the 2017 iMac.
If you can afford it, go with a SSD.
Actually, if you would put a SSD in your current iMac, I bet it would be a huge improvement, and you might be able to put off getting another Mac for a while.
Last summer, I put a SSD in my parents' Mid-2007 iMac after my father was complaining about how slow it was and asked me to recommend a new computer to him.
After switching out the HDD for a SSD, he said it feels like a new computer to him.
I am personally quite happy with the performance of my Fusion Drive.
Me too, but if I was getting an iMac today, I would probably stick with a SSD.
I have a 1TB Fusion Drive with the 128GB SSD in my Late 2012 iMac. It performs pretty well, but now I would try to stay way from spinning drives if possible.
(as far as I'm aware, only first gen 5K had 1TB Fusion with 128GB SSD)
For the iMacs, the 1TB Fusion Drives with the 128GB SSD were on the Late 2012, 2013, and 2014 iMacs.
The Late 2015 iMacs with the 1TB Fusion Drive switched to a crappy 24GB SSD, and Apple attempted to correct their mistake with the 2017 iMac by putting a slightly bigger 32GB SSD in them.
The 1TB Fusion Drive in the 2019 iMac still has the relatively tiny 32GB SSD.