If you're doing radiant with a condensing gas boiler, an indirect-fired HW heater is the "right" solution 99% of the time, from a total economic point of view. If you have a "real" heat load due to all of that glass, doing it with a real boiler is a far better solution than with any standard hot water heater. For loads under 25KBTU/hr you might consider a condensing tank type hot water heating solution. But if you're doing a full on solar-radiant, the solar designer would be your best guide on that. (The
HTP Versa series has versions set up for solar, but it's a hunk o' change.)
If your heat load per square foot it sufficiently low that you can do it all with 100F water or cooler, the Daikin Altherma heat pump would do well in a Sedona climate. Since the latent cooling loads are low in that climate you can probably use the slabs for sensible cooling as well.
But that's all a LOT of money compared to a mini-split. If your heating load is less than 30KBTU/hr, you might skip the radiant in favor of high-efficeincy/low-cost air-source solutions. In a Sedona climate a decent mini-split will have a heating seasonal average coefficient of performance higher than 3.0. (At 47F and higher outdoor temps they run about 4.0) At a COP of 3 each kilowatt hour buys you 3 x 3412 BTUs of heating, call it 10,000, which is about 1/9 of the heat you'd get per ccf or therm of gas in a condensing boiler, or 1/8 that burning it in a HW heater. If your electricity costs 20 cents/kwh that's equivalent to $1.60-1.80 gas (pretty expensive), but if your electricity is 15 cents it's like $1.20-1.35 gas (New England prices). If your electricity is 12 cents it's like a buck or so (maybe you're that cheap?). During the milder shoulders seasons with COP of 4 or better it's a major discount over heating with NG, but it's less fuel (or power) during those seasons.
But since it's also an air-conditioner, the systems-costs have to be factored in. I you took the 10 grand you were going to spend on pumps & plumbing & gypcrete and put it into PV, net metered the output of the PV can offset the bulk of the operating cost of a mini-split. It takes a sharp pencil and good energy-use modeling of the house in-situ to determine where all of the crossovers are. And ductless systems also need open floor plans or R25+ walls to really work well, or you'll have some rooms that run too warm/cool at some part of the year. They're VERY quiet compared to their reciprocation-compressor cousins though, utilizing scroll compressors (a rolling action, not a piston action), and continously variable speed drive (on both the blower and compressor for some units). They're about as loud as a refrigerator at full speed, but most of the time they'd be cruising a long at part-load, and whisper-quiet.
With a big investment in PV and heat-pumps for heating & cooling, the heat-pump water heater might make more sense than gas-fired.
A 16" on-center 2x6 with either cellulose or open cell foam comes in at around ~R14 after thermal bridging, bumping it to 20" o.c. with advanced framing only increases that to ~R15. Fill it in with 5" of clo$ed cell instead and you're still at only~ R16 & change, a performance boost that COULD have been had with 1/2" of EPS outside the cheaper cavity fill solutions. Adding R5 of exterior foam the R14-R15 wall brings it up to ~R20, adding R10 brings it to R25. You're correct that iso isn't the best choice under stucco, but 2" of EPS (R8) or 1.5" of XPS (R7.5) would be the ticket, using cellulose in the wall cavities for a low-cost R20+ (whole-wall) stackup. Alternatively, 1.5-2" of closed cell foam (R10-R13) on the exterior makes for a continuous and nearly assured air seal, albeit at a boost in price. (About 17 cents/r/square-foot compared to 10cents/R/foot for EPS.)
To air seal a studwall without spray foam you can either use the Huber ZIP approach (painted sheathing + tape), or caulk/glue the sheathing to the studs as you go, and lay a bead of caulk under the studwall plate etc. Spraying foam in the stud bays only treats 60-75% of the potential leakage, and there's no such thing as "too tight" (active ventilation is appropriate if you can get the place to test under 3 air changes per hour at 50 pascals in a blower-door test, which takes attention to detail to come anywhere near.)
For more on wall stackups & whole wall R, see:
http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls
and
http://www.ornl.gov/sci/roofs+walls/AWT/InteractiveCalculators/NS/Calc.htm
With a flat roof/no-attic, consider putting 4-6" of EPS above the rafters, below the membrane to make an R15-R23 thermal break over the framing, and filling the rafters with cellulose or new-school superfine fiberglass (Spider or Optima) between the rafters, and make the rafters 2x12s. That way you'll get a true R50 roof out of it, which will be important for both comfort & peak cooling loads, since you have no convective cooling of the roof going on and the peak temps on the surface will be high. New-school blown fiberglass might be a better choice here, since without an attic a roof leak might go undetected too long if you used cellulose. Open cell is an option, but it'll be more expensive, with very little if any performance gain, since a membrane-roof is already air-tight. Spot sealing the ends of the rafter bays with foam is still a good idea though.
Also use a
CRRC rated "cool roof" material for the finish roof, which will take a significant edge off the peak & average loads.