Is this with 128gb SSD? I'm wondering if the 2TB with 128gb SSD would be enough.
Yes, this is with the original 128 GB SSD.
And no, it was not enough, as you would notice a slow down on things that were not done often. For example, a few years ago, I was playing WoW often, and the load screens were very short. I have stopped playing, but still pay for a few accounts for my daughters. Logging into the game now, the load screens are much longer.
Another example is restarting the iMac. Since I leave my Mac running for weeks/months at a time, if I ever have to restart, it is very slow to do so.
But, I am recommending a SSD vs the Fusion drive for other reasons too. One being that the Fusion drive has moving parts, and is more likely to fail.
I had the HDD fail in my iMac's Fusion drive, it was partially failing for about a year before it fully died. The Apple Store refused to replace the drive because it kept passing their hardware tests. Luckily, it died completely 12 days before the Apple Care expired.