Can you rank the importance of upgrades for editing.
Would it go something like this:
1. GPU
2. Fusion Drive
3. i7
4. Bigger HDD
Is that about right? (I'm leaving out RAM because that goes without saying).
I would switch i7 with the Fusion Drive. GPU and CPU are the most important, especially on an iMac: you can't upgrade those later. You can always get a Thunderbolt HDD or SDD later to add more speed and/or storage.