Link to map with average water temps. You'll probably want to dig deeper to try to find the minimum water temps though. (I remember reading something a long time ago that the minimum water temp is in late spring so, if all else fails, you could measure your water temp now & get close.) Also, if you're doing a new build & will have your own well, my experience is that well water is always colder than city water.

As far as flow rate, new faucets are limited to something like 2.2 gpm so you can go from there & adjust for however many faucets (that you anticipate to be operating at the same time) that the heater will serve.

Oh, and don't forget to allow for dishwasher hot H2O demand.

Here is a link that might be useful: Average Shallow Ground Water Temps