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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
Ugh, really? I was originally looking at getting the Nvidia GTX 1060 and an Akitio Node plus a TB3>TB1 adaptor, a few forums were pushing me in that direction and budget is close to what I'd be willing to spend...