If you want a high performance rendering system don't acquire an iMac.
I have two macs and the rest of the systems are PC, PC is for 3D work, mac has really low 3D power compared to a PC, you can't have a Titan inside a iMac for example, maybe (and just maybe) in a Mac Pro, but not inside an iMac, also you can't have 64Gb of RAM AFAIK, you can't modify your system and renew just the CPU for a more powerful one in the future.
iMac is not for 3D IMHO
Cheers!