I agree with Tim that the Fusion drive is great for OS caching but doesn’t give you much more improvement in editing.
I’m currently looking at a variety of setups with SSD drives and what I’ve commonly come across is that three drives is the key, minimally. If you have a low capacity SSD, your scratch will fill it very quickly and this could lead to you having to delete peak files etc, which could impede your edit workflow.
An internal SSD would make the applications run lightening fast. I’d suggest getting a USB 3 drive for your scratch as these are fairly cost effective.
Another option for your rushes would be to get a BlackMagic MultiDock. This allows for Thunderbolt connectivity with the option of docking four SSDs if you want to build them up over time that you can use for your source media. Also, it’s worth thinking about the SSDs that you buy. Samsung offer a five year warranty with their disks.
You’re only going to be as fast as your slowest item so take the graphics card into consideration too as Premiere and After Effects are very graphics card heavy. Look at graphics cards with CUDA cores to enhance your processing speeds and obviously a fast processor will also help, as well as a good serving of RAM.
Even if you’re only outputting for the web, you’re still going to have to handle the demands of 4K files in reading your source media, you could be outputting a 200×113 file size but your machine will still have to transcode those 4K files.
If you have the budget to get five iMacs, I’d consider whether you truly need five machines that all need a bit more work or whether you should get four and make them a better specification.
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Post Production Co-Ordinator
Opinions here are my own