This really should have been a new thread, eh? (It has nothing to do with Navien, other than that Navien doesn't void the warranty when their units are used for space heating.)
Every reasonable heating system design starts with a good heat-loss estimate/calculation.
If you build the radiant in to a concrete slab, it doesn't need a buffer tank.
There are cheap ways of doing radiant with a tankless, but if your design condition water temp requirements are low enough, there may be better options.
In climates as mild as Bend, for new construction, you have the option of spending the money on high efficiency building envelope rather than a high heating system. If you target design condition heat loads to be <<20KBTU/hr (not tough to hit for 2000-2800' house in Bend's climate, but it has to be designed), at which point you can have your cake and eat it too, since you can then heat & cool the place with a highly efficient ductless mini-split. (Even if it's not as cushy as radiant floors...) If you're not on a gas main and would be otherwise be using propane, it would be practically INSANE not to use an R410A refrigerant heat pump for primary heating in that climate. (If propane-fired radiant, even R410A air-to-water air source heat pumps start looking pretty good in terms of 10 year NPV compared to condensing propane, even though they're far more expensive than mini-splits.)
Consider building to Zone-5 specs in the table 0.2 on p10 in this document, for starters:
Note that those R values are not center-cavity, but rather "whole wall" R with thermal bridging of the framing included. An R30 wall isn't a 2x10 studwall with R30 batts- when the thermal bridging of all framing is factored in that would come out at ~ R21-R22 for homes with "typical" or "average" framing fractions. But a 2x4" wall with R13 batts or spray cellulose + 3.25" of exterior polyisocyanurate rigid-board is ~R30, with typical framing fractions.
Air infiltration is a LARGE factor in total heat load- designing a continuous primary air barrier on all 6 sides of the cube and having a Konstruction-Kommandant to enforce air sealing is critical, as is blower-door testing & remediation on the main shell as soon as you have the windows & doors installed. Typical pretty-good construction comes in at ~ 10 air changes per hour @ 50 pascals pressure (ACH/50), the standard leakage test. The IBC 2009 standard specs out 7 ACH/50 or less, which is usually achievable as a post-construction (read "post test-failure") retrofit. But to be very efficient you need to be under 3 ACH/50 and under 1.5 is better, and relatively easy to hit, if you have a plan and execute on it.
Air sealing is by far the most cost-effective envelope performance upgrade you can do- a well insulated wind tunnel is a waste. Put a bead of caulk or acoustic sealant under & between stud-wall plates, foam seal & gasket foundation sills, caulk every sheet of structural sheathing to the studs, etc etc. It's cheap & quick, but it has to be consistent. On upper floor ceilings use OSB or ply on the underside of the joists/truss-chords (you'll need it to hold up the 20" of cellulose without bowing), and detail it similarly as an air barrier. Don't mess around with stuffing fiberglass in around window framing either- use the appropriate compliant foams.
Only use insulated doors. Don't use sliding doors- they all leak like crazy with age (some even when new.) Swinging patio doors/french doors can be made to seal better. (But see notes about minimizing glazed area.)
Minimize the total glazed area except where passive solar gains have been site-simulated and optimized. Every square foot of U-0.34 pretty-good window is an R3 hole in your R30 wall, with literally 10x the heat loss per square foot. Size & locate them for daylighting & egress needs.
Use fixed (non-opening) windows where you don't absolutely need to open a window- they leak LOT less air. Where they must open, use casement & awning types, since they leak less air than double-hungs & sliders, and they give more egress & ventilation cross-section per square foot of glazing too.
Avoid recessed lights, particularly those that would penetrate into attic or cathedral-ceilings. Even IC rated air-tight versions aren't usually all that air-tight, and make thin spots in the insulation.
If taking the foam-clad framed building approach, a LOT of money can be saved by using reclaimed roofing insulation from commercial re-roofing jobs. An overcoat of R18-R24 iso or eps comes in at well under $5K for most reasonable-sized houses, which roughly triples the whole-wall R-value of a 2x4 fiber-insulated wall, and more than doubles that of a 2x6 wall. (If virgin stock it could easily hit $12K+.) Going to an air-tight R30 with glazing reduced to under 15% of floor area (as opposed to the ~18% new construction average) can cut the heat load of a house down to 1/3 or less of a typical-leakage typical glazing fraction code-min house, without having to live in dank darkness.
To see what foam clad timber frame looks like, check out the retrofit the section titled "An architect works on his own house":
(Note the 1-part expanding foam in the pictures that seals the seams of his 6" of reclaimed iso board.)
Other foam-cladding retrofits can be seen here: http://thousandhomechallenge.com/case-studies
If you ARE on the natural gas grid, running low-temp radiant with a condensing water is roughly comparable to heating with a mini-split for a low-heat load house, but you pay quite a bit up front for that extra-cushy warmth underfoot. There's no payback on it, other than the "aahhhhhh" factor when it's 10F outside.