That sounds like a good deal to me.
I am sure that this machine will be good for over 3 years.
No one knows when the new iMac will be out. It could be next week, or it could be in 2020.
There are threads earlier this year with people asking if they should wait for the 2018 iMac, many people thought that there would be 2018 iMacs last Spring, then summer, then fall, and it still has not happened.
I said last winter that there might not be new iMacs until 2019, and it looks like this is correct.
You can return the iMac and wait for the new one if you want, but I would just take the deal you got. Even if new iMacs came out tomorrow, it is not like your iMac would be instantly obsolete.
Also, with the way Apple is going if their HW, the new iMac, if it is a redesign, might have some undesirable features on it, like soldered SSD or RAM.
[doublepost=1540991120][/doublepost]Configuring it on the Apple Website, that Mac would be $2,200 new. $800 off is a great discount.