I haven't been checking the selling market to know if $950 is a fair value for your system, but I will say that I recently considered trading in my late 2015 27" retina iMac that has the options for maxed out CPU and graphics, along with a 512 GB SSD, along with an upgraded 32 GB of RAM. Apple's trade-in value was only around $760.
The computer you're looking at through B&H strikes me as a poor proposition - I'd imagine a system like that would show up in Apple's refurbished store for less. But again, I have to admit that I haven't been watching the prices all that closely to say for sure whether it's a good deal or not.
The big question I can lend some thought to is whether it's a worthwhile upgrade, and to that I have mixed feelings. If your current system is bogging you down then the answer is yes, but not because of the base system, itself. Rather, our 2015 systems are limited to Thunderbolt 2, and things are really taking off with Thunderbolt 3 (first introduced into iMacs with the 2017 systems). Just as an example of what you could do with it:
1) If your graphics card is bogging down, you could get an external GPU. This will cost you a few hundred dollars but it's an upgrade you don't need to do immediately, and it can give you more power than anything you could buy in the standard iMac from Apple.
2) You can run external SSDs at speeds faster than Apple's own internal SSDs. Needless to say, the cost per storage size is also more favorable if you do it this way, as opposed to buying through Apple.
The only downsides to those approaches are having things hanging off your ports. If you're a minimalist and like to just see the iMac on your desk then the aesthetic may possibly be ruined. If that doesn't matter much to you, then you're gaining a lot of potential expandability.
If your current system is running well enough for you then I'd say keep it, and upgrade later. For all the expandability of Thunderbolt 3 it remains to be seen just how far that can take systems and extend their usable life, or whether it'll just be another factor that only gives you an extra year or two. (For what it's worth, there are no rumors about Thunderbolt 4 at this time - the big development for Thunderbolt is that it's being handed over to USB and will technically merge with USB, which will then become known as "USB 4." That shouldn't impact any Thunderbolt 3 peripherals now or in the future.)
