Agree with getting an Apple SSD to start, it's the fastest. The SSD in the Fusion drives is NOT as fast as the SSD only models. I have a late 2013 model that started as Fusion, replaced the HD with SSD myself, not much difference in speed gain at all. The PCIe/NVMe connection is a game changer in speed.
Just ordered a loaded 2017 iMac, except I will be adding the RAM myself. This one has the 1TB Apple SSD and NO Fusion this time.
From personal experience and my buddy's iMac:
Mine is a loaded late '13 27" iMac....I installed a Crucial 2TB SSD.....i7 3.5 and 32 GB RAM.
My buddy has a refurbished late 2015 27" iMac, i5 3.2 and 16 GB RAM.
I use a lot of Lightroom, Photoshop & FinalCut Pro daily for work. Tested a bunch of stuff on his and it's faster than mine without lag. For example, in Lightroom, 8 large RAW images export to Photoshop in exactly 1/2 the time on his than mine. On the Black Magic test, his is 3 times faster than mine. In practice, his is twice as fast and adjustments in Lightroom happen in real time instead of a second or two lag.