I use hydronic heat in my hone in Northern Arizona. I chose to not
install in-floor heating, although that could work just as well as my
in-wall fan convectors.

Most on-demand hot water heaters are not rated for use as a boiler (ie
they're not intended for continuous use) but many will work just fine.

There are a number of different plumbing configurations possible, but
you'll need at least a circulator pump (or more than one). If the
system is intended to provide both domestic hot water and hydronic
heat, you'll want to install a heat exchanger to isolate the two
loops. Also, be aware that most in-floor systems want to keep the
water temps below 109 which is often a bit on the cool side for
domestic hot water.

I use a Takagi K2 which does require electricity to power the control
board. On the other hand, I burn less propane that way. If I were to
do it again, I'd probably opt for a heating oil based system paired
with a wood fired boiler.