Wait if You Keep Your Mac for a Long Time
Do you keep your Mac for a long time? If there is a good chance that you will want to use the same iMac for the next four years, you should wait and see what happens by mid-2020.
The current iMac is good, but waiting a few months, could lead to a new design, more features and another year of software support down the line.
I would wait and see what Apple does with the new iMac models before you buy one.