Gas tankless heater, a deep well and a good generator equals = showers for the neighbors when the power is out.
I’ve been using the same Bosch propane tankless for the past 18 years. The plumber that installed it is out of business and there are no Bosh service companies willing to travel to our house. I had to replace the regulator once and have cleaned it with vinegar several times due to our water softner not always doing its job.
BTW: Our tankless is mounted with a roof vent in our laundry room . Our neighbor’s heater was installed in a separate room outside. The neighbor’s heater lasted one season and we rarely see -10 weather.
Since you asked: find a good plumber and purchase what they support, pay for the best unit available and still learn how to fix it yourself. If there are more than two of you in the house, then I would make sure that the one that you get can handle multiple sources of outflow at the same time. ( i believe most do now)
You may also want to plan on a backup heater:
I’ve used an Ecotemp l5 in our outdoor shower for the past 3-4 summers. It is not terribly reliable: the flame will cut off periodically. For the $105.00 price, it is a small inconvenience.
I purchased it here: https://www.sportsmansguide.com/productlist?k=water+heater