200 amp service is fine. It will probably use 2 or 3 40A lines.
I have a tankless in florida on a 150 amp service, but we have public water, so it starts out warmer.
You need to measure the well water temp, and size it based on demand (Gallons per minute - fixtures used at the same time) and water temp input.
I have the ecosmart 27, very happy with it. Below is sizing chart:
