Personally, I would switch the Dixie for a Plaits (the sounds of the Dixie are covered in just the first three synthesis modes on the Plaits: variable saw and pulse, hard sync, FM, waveshaping. And then with Plaits there's the formant, additive, wavetable, wave terrain, vowel, granular, particle, resonator, drum modes, etc. not to mention the firmware upgrade where you can add your own sounds)—big value for a little case. I promise.
And I would switch the Source of Uncertainty for a Wogglebug, which markets itself as an expansion of the Buchla design anyways. And it's like half the size. That frees you up 10hp for an additional module—you have six slots in the 64X.
With daisy-chained cases giving you 128hp, and this bit you wrote:
"I'm more interested in setting up an exploratory tool for playing with raw sound. My creative output thrives on limits and finding ways around those to find unorthodox methodologies and sounds."
I would recommend a Morphagene or Arbhar. It might also be a good idea to get a module that you can send to your pedalboards. Something like: ALM006 - S.B.G.
As for the rest, you've got it right: "I could get my hands on a few good pieces, learn what I've got, what I lack, and then build out the second case based on that."
Learn on the job and you'll learn fast.