Colin, I don’t know about Keyshot and don’t use it (I’m using ZBrush for jewelry). However, I’m using an iMac 27" Late 2013 with a 3.4 GHz Core i5 that works extremely well. I do, however, have 16 GB of RAM. I would consider upgrading your machine (it’s really easy…I think I got the RAM from Other World Computing). I am also using the 64-bit version of ZBrush on this configuration, and it seems like the fan doesn’t come on as frequently as it did with the 32-bit version (as a Mac user, I couldn’t have cared less about 32/64-bit software until ZBrush, and couldn’t have told you if I had a 64-bit machine).
One thing I would watch out for…a lot of the tutorials online will bump up to 6-12 million points very quickly, and your machine will seem sluggish. Pay attention to the ZClassroom videos on Dynamesh (especially the ones in the ZBrush Core section with Joseph Drust…his are about the best explanation I’ve found), and get your rough form completed in Dynamesh before going in for the details.
But your iMac will work well. I also run it on my i7 MacBook Pro from 2012, and as long as I use that technique with Dynamesh and keeping the point number low, then switching to the finer details, I have no problem with that machine. I thought I might have to buy a new one, but it wasn’t necessary.
You’ll have a blast with this…also check the YouTube videos for AskZBrush. 
