Having owned a $2.5k iMac myself, I am going to say that it's not worth it, especially if the only thing you're going to be using it for is basic photo editing. Reason I say this is because you don't get that much for $2k. Old GPU, 8GB RAM, non-SSD drive... The repair costs are going to be through the roof if anything breaks due to a lot of the stuff being proprietary.
I'm currently waiting for NVidia to release drivers for the new Pascal cards, at which point I'll probably end up building a hackintosh for around $1.5k. The screen won't be a 5k, but the hardware is going to be running circles around all of the current iMac's.