OP:
You didn't tell us about the most important thing.... your budget.
Do you want a Mac just to experiment with, or are you looking to move the bulk of your "computing needs" over to it?
If the latter, and if you can afford it, don't waste money on a 10-year-old Mac. It's just "too far behind" now.
If you want something that's going to "carry you into the future", then either look at a new 2019 iMac, or perhaps at an Apple-refurbished 2017 design iMac.
If you have a brick-n-mortar Apple Store nearby, go to it.
Or... a lot of Best Buys also have an Apple section.
Look at both the 21" and the 27" models.
If you can afford it, buy the 27" and don't look back.
DON'T get ANY iMac with only a 1tb fusion drive.
Get at least a 2tb fusion drive model.
Or, get one with an SSD inside. Be aware that you cannot buy these in stores -- they must either be special ordered "as new" from Apple, or you can also get them from the Apple online refurbished store.