I don't know about the OP but the least expensive 27" retina iMac (new) that I could tolerate would be a 3.2Ghz with 2TB Fusion Drive, and that is $2200 -- $1000 more than the 2013 he mentioned and lacking SSD and 16GB RAM. There are some factory refurbished 27" retina iMacs for less but my advice is don't get one without Fusion Drive at a minimum, and the 1TB Fusion Drive has such a small SSD cache I'd be afraid of that. Still, the ratio of HDD to SSD on the 1TB is 41:1, vs 23:1 for a 3TB SSD, and the 3TB works very well. So maybe even the 1TB Fusion Drive would be usable.
I used a 2013 27" iMac for a long time, then side-by-side with a 2015 27" retina iMac. I thought the non-retina was OK until I got the retina, but now I'll never get anything else. For whatever reason the non-retina screen just wasn't that crisp on text -- it was even worse than my old PC monitors. I'm typing this on my PC with an old Dell U2412M monitor. It's $250 but is significantly sharper than the non-retina iMac 27 I had.
I have a 2013 MacBook Air and it gets the job done but the screen is pretty fuzzy compared to my 2015 MBP. Once you've used a retina screen it's hard to accept anything else. That said, $1200 for the configuration the OP mentioned is very good.