Yeah, the iMac is an older model; obviously so is the PC.
The new iMac's 5k display is quite notable. Although, there are many sceptics who question the need for such a high resolution; is the human eye even capable of processing such a high resolution? And then there is the issue that 5k content isn't going to be adopted quickly; 4k isn't even a commonly used resolution yet. What's the good of such a high resolution display is there is little content to actually make use of the extra pixels?
Using your build, let's replace the 5k display with a 4k display at costs $500. The total for the PC would then come out to $1672.
http://www.amazon.com/gp/aw/d/B00IE...&dpID=41lK46eWlpL&ref=plSrch&pi=AC_SX200_QL40
I suppose it depends on how much you value 5k resolution; is it really worth near $900? Personally, I would wait for 5k content to be used more first and for the prices to drop, which will inevitably happen in the following years.