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What about a 2014 mac mini? Cheap as chips, it'll be a bit slower but should still be very usable and you can upgrade to an SSD in (though you can't upgrade the RAM). OSx has pretty decent virtual memory management which uses the HD as virtual RAM, so if you run out of memory it'll treat the SSD as an extension to the onboard RAM. My 2014 mini pootles along fine :)
At a later stage you can upgrade to an eGPU if you want which will give you all you need I think.
Yeah but that's such a minefield, your posts kinda put me off! 😉