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In theory a modular solution with an eGPU is the way to go, but in practice, OSX may not be capable of the blistering performance that an eGPU can deliver and especially not over TB1. Give me a chance to play with my set up and I'll report back.
Honestly there's not point going modular and scrimping on every component - you should overspec the housing so that later you can upgrade the card, otherwise upgrading will be a massive compatibility PITA
Also @chrisbmx116 is right, check which cards will drive TB1 monitors and how?
For your needs I'd be temtpted to get a Mac Pro from the refurb store, they were massively discounted by Apple recently so are very good value right now
Hmmmm that is a pretty small budget to play with but doable. Maybe go 560 over 570 and it’ll come in at that. A desktop 560 is notably more powerful than the 560 pros in the bigger machines so you could upgrade to a 13 inch when the time comes and save money over the 15. FCP X uses egpu power but not sure about CC. Oh, more importantly I don’t think your monitors will work with an egpu.