I own quite a few iMacs, the ones I own from the 2011 to 2013 models are:
Mid 2011, i5 (2.7GHz), 6770M 512MB, 32GB RAM
Mid 2011, i7 (3.4GHz), 6970M 2GB, 32GB RAM
Late 2012 i7 (3.4GHz), 675MX 1GB, 32GB RAM
Late 2012 i7 (3.4GHz), 680MX 2GB, 32GB RAM
Late 2013 i7 (3.5GHz), 780M 4GB, 32GB RAM
The only one listed above I bought new was the Late 2012 with the i7 and 680MX, and that one is a beast. The 2013 definitely faster than the Late 2012 with the 680MX, but not nearly the difference from the high end 2011 to high end 2012. I now use the 2013 more than my old favorite, but I only just recently got that one a few months ago.
I personally think that $300 for only the i5, 675MX, 8GB of RAM is a little too much, but then again, maybe I have been lucky in the finds I get.
I got the Late 2012 i7, 675MX iMac last year for only $50 from a friend, but to be fair, the internal HDD broke, so it wasn't technically fully working.
I for the Mid 2011 i5, 6770M for $100 on a local forum back in 2020, it was perfect, but needed a good internal cleaning. It was just too slow. I tried swapping the GPU for a K4100, but I think the used GPU fried. I might get around to looking at it again one day.
The Mid 2011 i7, a friend pulled it from a dumpster, and it didn't work, so he gave it to me. I replaced the HDD and baked the GPU and it worked great. Not only that, it ended up being a top of the line for that year.
Again, I couldn't picture paying $300 for the specs on that iMac, especially with the M1 Mac prices being so close, but I fully understand that the OP could be in a totally different position than I am in. I would keep looking, but if you are in a pinch and need something right away, maybe it might be worth it to you to take your friend's iMac for $300.
would imagine the 675MX is not that much slower.
The 680MX is a beast.
I have two Late 2012 iMacs, both with the i7, but one with the 680MX and the other with the 675MX, and the 680MX has about 40% higher fps than the 675MX on WoW. It is a hell of a upgrade for gaming.
Not sure how that translate to Pro apps like Photoshop and Premiere. I assume the 2GB of VRAM in the 680MX would give it a significant performance increase over the 675MX with only 1GB VRAM.
I tried to look up some Pro app benchmarks comparing the two, but could only fine gaming ones. Here is a huge list of comparisons, the 680MX typically has a 30%-40% increase fps over the 675MX:
NVIDIA GeForce GTX 680MX vs NVIDIA GeForce GTX 675MX - Benchmarks, Tests and Comparisons
www.notebookcheck.net
That said, I am unsure if GPU performance has as big of an impact on graphic design as RAM and CPU would.
I got a top of the line Late 2013 iMac in eBay for $250 a few months ago.
Correction on this one, I looked it up on my eBay account, and I actually paid $350 for this one on eBay back in Feb 2022.
I remember now, prior to finding the 2013, I was looking at a Late 2012 iMac, with the 3.2 i5, 680MX for $150. It had a damaged display, but I was going to take the display off of my Late 2012 with the 675MX and put it on the eBay one.
But, I found the Late 2013, i7 (3.5GHz), 780M 4GB, 32GB RAM instead. Even @ $350, the Late 2013 ended up being a pretty good deal even with the HDD not working in it. Although, it could have been DoA and I would have been screwed.