Electric tanks have very low standby losses, and maintaining them as buffer tanks will eliminate the most egregioius of tankless "personalty problems).
DO insulate 100% of the distribution lines from the Rinnai to the tanks with 3/4" wall closed cell foam pipe insulation, or there will be substantial amounts of energy abandoned in the lines. Set the output of the Rinnai to at least 5F above the setpoint of the tanks.
Setting up the tanks with a recirculation loop, but controlling the pump with the aquastats on the tanks (disconnect the elements and drive a pump instead) would make it an all-gas-fired system. Done right it would keep the Rinnai from short-cycling on small draws, extending it's service life and operational efficiency. (insulate the return line as well.) With a recirculation loop you'd set the output temp of the Rinnai fairly high so that it hits something like the maximum modulated firing level at the beginning of a burn with the cooler water of the return line entering, dropping to half or so when the tepid water at the bottom of the tank hits. That way it'll modulate nicely during bigger draws, but still have decently long burns when in temperature maintenance mode.
Whether the crawlspace is ventilated or not, it should be an indoor model if it's inside the structural walls. FWIW, in NC you're better off converting any crawlspace into a "conditioned crawlspace" from a humidity & wood-rot control point of view. This would also protect the Rinnai from freeze damage. This involves sealing any ventilation (or converting them to operational windows/hatches), putting down a ground moisture vapor retarder (10mil poly sheeting) and foam-sealing the foundation sill & rim joists. (Even better, put a couple inches of rigid foam insulation against the foundation with Z-flashing at the top at the foundation sill for ant/termite control.)