Since it's primarily a summertime use situation, the odds are the incoming water will usually be at or above the deep-well water temperatures, so 48-50F seems likely:
If you have just one shower load at time even a ~100KBTU/hr unit would do, but a ~140KBTU/hr Takagi TK-Jr or Rinnai RV53P or similar would be enough tankless for 2 simultaneous summertime showers (I'm assuming you won't be showering outdoors in April very often...)
Brand wise- it's better to go with whatever has more local support & installer expertise. The big 3 would be Noritz, Rinnai, and Takagi which combined probably take more than half the US tankless market(?). Rinnai is the world's largest manufacturer of gas-fired appliances, and have pretty good north American support. Noritz has some of the nicest units in terms of tight temperature control at low flow. Takagi is a solid "value" version with reasonable reliability & support, and for similar size & features usually (but not always) comes in a few hundred below the others on installed price.
What you gain with a tankless is several square feet of floor area, but you get to put up with a few quirks as well. The "cold water sandwich" is the slug of unheated water that made it through the tankless during the delayed ignition cycle, putting a bit of cool or tepid water in the pipe between more fully-heated sections. Some find it pretty annoying, but that's more of an issue in the cold-water months in colder climates than yours.
Navien has pretty good specs at very attractive prices, but (apparently) not as tight quality control and (apparently) even less control over the expertise of the installers. I know people who have used them problem-free for years, but if forum posts and complaints about spotty support are any indicator, unless your installer swears to stand by it you may want to look elsewhere.
With any tankless be sure to have it installed with ports and isolating ball valves on both the cold and hot sides to be able to drain it for the winter (for freeze-control on the heat exchanger), and to descale it if it starts to lime up (a common problem in hard-water areas.)