I ended up getting the Coway BA13BR. I went to the Bidet Shop location in Auckland to see it in person first. It cost $1,490 buying it in person (versus $1,890 on the web site). They have a book of toilet suite models to help you confirm if the round or elongated model is right for you. Friendly, but not overly concerned about backflow - the demo setup was rigged to the internal supply of a back-to-wall unit using the kit they offer on the web site. I’m guessing because the BR13B has the air gap that is technically okay.

Plumber installed with a valve based backflow preventer and the in the box valve. We discussed options and they figured that would be sufficient. It sounded like they get asked to install devices like the handheld hoses without the homeowner caring about back flow so they were impressed that I was asking about it.

Total cost for unit, electrical, and plumbing - $2,714.84. Having a cleaner bottom - priceless.