I'm very sorry to hear that the house is gone! (At the same time, I'm glad to know that you're still with us!)
ZIP is an OSB product that is manufactured with a pre-applied weather resistant barrier (no housewrap or #15 felt necessary.) It's a decent product, but not necessarily the way to go. One advantage of ZIP over other OSB is the ease of air-sealing the sheathing layer using their proprietary tapes to seal the seams. While sealing the seams is necessary, it's not sufficient. The framing still needs to be sealed to both the sheathing and the foundation.
In US climate zone 6 a 2x4/R13 + R10 continuous insulation would meet IRC 2015 code-min per TABLE N1102.1.2 (scroll about half way down the page
) and would have plenty of of dew point control without need of interior side vapor retarders tighter than standard latex paint per Table 702.7.1 (about a quarter of the way down this page
.) With a 2x4/R10 + 10 stackup you could use either damp sprayed cellulose, dense packed cellulose/fiberglass or open cell polyurethane foam, in the cavities, which ever is cheapest (probably damp sprayed). Cellulose would offer the best moisture resilience, 1.8lb fiberglass or 0.7lb open cell foam would have a (very slight) performance edge.
Closed cell foam between thermally bridging framing is a waste of good foam, and very expensive- not worth considering as cavity fill. Spend the foam budget on continuous exterior foam where it's performance isn't being cut off at the knees by R1.2/inch framing. The "whole-wall-R" of a 2x4/Rxx + R12 wall is about R22-R23-ish, independent of cavity insulation type.
For the exterior foam in your climate 2" foil faced polyiso (labeled R12-R13) would be the easiest to build and is easy to air seal with foil tapes. An alternative would be 3" EPS (labeled R12.5-ish) which would outperform the polyiso in mid-winter but underperform it slightly in summer. Polyiso needs to be derated to about R5/inch for heating season performance in your climate in this type of application, since it underperforms it's labeled R at the cold temperature extremes. EPS outperforms it's labeled R at the cold temperature extremes. A 2x4 wall with 2" of exterior polyiso is the same wall thickness as a 2x6 wall, but has only 2/3 the heat loss of a 2x6/R20-23 wall (which meets state code for zone 6 in states still operating under IRC 2006 or 2009), and is inherently far more moisture resilient.
If you're thinking about double-stud walls or other high performance walls, take the time to ponder this document.
(including the hygric simulation analysis.) A double studwall with VENTILATED siding and and 2-mil nylon (eg Certainteed MemBrain, costs less than 15 cents per square foot) as the interior vapor retarder under the interior gypsum and detailed as an air barrier it almost wouldn't matter what the cavity fill was. But cellulose would offer the most resilience, as well as a measurable thermal mass benefit once you're at 6" or thicker 3.5lb cellulose. You'd also do OK using "vapor barrier latex" as the vapor retarder, but in your climate zone there still be some risk if with only a standard latex paint & vented "rainscreened" siding alone.
If you're considering taking it to Net Zero Energy, review the whole-assembly-R values & U-factors in Table 2, page 10 of this document
If you're going for R25+ whole wall and smaller higher performance windows, with a Roth panel or WarmBoard approach you'd very likely be within range of a 2 ton Chilltrix, but keep running the load numbers on the house as you tweak the design. Now is the time to register and download a copy of BeOpt
and get familiar with how to use it (I'm not an expert on it myself- yet), which makes it much easier to play "what if?" games with design changes to figure out both energy use and peak loads, both of which are important. But you can still do a lot with an I=B=R type spreadsheet heat load calculator, if tools like BeOpt aren't your cup o' camelia sinensis.
With a high-R house that isn't quite covered by the output of a chiller-driven radiant floor, cold climate mini-splits sized for your cooling loads would more than cover the difference (or even the whole thing), not need for propane backup. As a "Hail Mary" back up for power outages an EPA rated wood stove may not be a bad idea, and for a high-R house it doesn't have to be very big at all.