Personally I would never go with a Fusion drive system. What that is, It is a combination of both a SSD (Small, 128 or maybe 256 GBs) and a Larger spinning hard drive. They are combine together to look like One Big Drive to the operating system. If either fails the whole system comes down and you lose all your data.
The OS decides what get placed on each physical drive, you as the user have No Say in that.
I suggest you get the largest SSD that Apple offers and then add on Thunderbolt or USB 3 (or USB C type if Apple is offering that type of connection on the currently available iMac's) external drives for extra storage and backups.
You might also look into a multi drive NAS system for even more storage that you access over your home network.
I gave up on Apple about 2 years ago. So I don't presently own any Apple products but I am fairly familiar with what they offer. That is one reason I gave up on them. They don't offer what I need and want in a desktop computer.
The trick is to keep just your catalog and most recent images on the SSD drive and to put the bulk of the images on a big hard disk. I use a MacBook Pro as my main machine generally hooked up to a high quality monitor. I keep the catalog on my internal disk and have only my most recently shot images on the same disk. Most of my image files are on an external disk (a nice fast little RAID array). It is just as useful to use a simple external spinning hard disk if you don't want to go full-on RAID array but then you need to be more vigilant in making backups of the external as contrary to RAID you will lose your images if the disk fails.
This strategy gives the best of both worlds in my opinion. I have a laptop that will travel with me when I want and I can ingest images at night when I am not out shooting and at home I just hook it up to monitor and hard disk and I have a bluetooth keyboard and mouse and I am off to edit on a nice speedy editing station with exactly the images I need and I didn't need to shell out MAJOR bucks for the biggest SSD Apple offers and still run into space limitations. I spent most of the money on maxing out the RAM and processor on the machine as those are far more essential for Lightroom speed.