Now I'm stepping back and thinking about needs versus ideals. Here in Seattle, as my local compatriot @Marial can attest to, most folks don't have air conditioning in their homes. It's largely not needed. It would be nice to have on about 10-days out of a given year, but the other 345 it's largely not necessary. I'm now wondering if a HEAVILY insulated structure in Seattle actually needs air conditioning, or if it can get by comfortably with only a heat source for the colder months, and a way to exchange the internal stale air with external fresh air sufficiently to meet code and not smell like an old band van. Sure it wouldn't be ideal, but it would keep costs down to the point where they're feasible.
The quote was so far beyond what I was expecting that it's forcing me to re-think things in a serious way. I've verified with some folks that the quote is actually not far off of realistic. I could cut it down by eliminating the storage vestibule and only needing a single interior unit. I could also possibly go with some lower spec units (they might be noisier, sized smaller so they have to run more frequently/intensely, or possibly both) as well. That said, I don't see these things as keeping costs down far enough. I was expecting to spend another $15,000-20,000 to finish this job. With the current HVAC quote I'm now just over $30,000. At that level I'd be getting into home equity line of credit territory, or possibly setting up my studio as a proper business (i.e. not an "under the table" outfit") and securing a small business loan. Either route is scary, and hopefully I can mitigate that by what I described in my first paragraph.
Let's see what code and some HVAC experts have to say about my no-AC approach. In a sealed up, as-close-to-airtight-as-possible, no windows, door closed environment it might not even be feasible. We'll see...