So does the note about 2014 and later models not supporting target display mode meant I can use them for a second monitor?
The 2014 Retina models, and the 2015+ models WILL NOT do Target Display Mode according to Apple.
The Late 2009 - Mid 2014 (non-Retina) models can do Target Display Mode.
The most important thing to her it that they match as far as size and shape so the look identical sitting next to each other.
From the front, I believe that there are no noticeable differences between the Late 2009 through the 2017 models.
I noticed the newer models have a much smaller profile from the side and want to get the oldest one possible that still looks the same.
This started with the Late 2012 models, so if you want identical form between the source iMac to the second one, then you will have to start with the Late 2012. From the side view, the Late 2012's look like the 2017's.
Maybe someone could chime in if I am incorrect about any of this, as my newest iMac is the Late 2012.
[doublepost=1544211147][/doublepost]
From the front, I believe that there are no noticeable differences between the Late 2009 through the 2017 models.
This started with the Late 2012 models, so if you want identical form between the source iMac to the second one, then you will have to start with the Late 2012. From the side view, the Late 2012's look like the 2017's.
Just a side note, depending on the position of the iMacs, the user might not ever notice the thin models from the thick ones.
I have a Late 2012 27" iMac. Due to the design and position of my desk that the iMac sits, my children have never seen it from a side profile.
A friend of mine pulled an iMac out of the trash that wouldn't boot up and asked me to look at it. It end up being a max out, BTO Mid 2011 27" iMac. I got it working, and it runs really well.
Anyways, while my girls were at school, I was working on the iMac on my one daughter's computer desk. They came home and saw the iMac, and due to the position of the iMac, they could clearly see how thick it was.
For a few days they thought it was my Late 2012 iMac until one of them saw mine on my desk and was confused about the second iMac. I told them the story about it being in the trash and asked them why they thought it was my Late 2012 iMac. They swore it looked exactly the same, until I rotated my iMac for them to see how thin it was, and they had no idea.
My point is that from the front, your wife would probably not know they were not the same thickness.