You need to know the total gallons per minute, and gallons needed per hour, and the temperature you need. Then, any of the water heater manufacturers will have guidance on selecting a unit.
Get out the checkbook, because this won't be cheap. A 100 gallon, 200,000 BTU unit will run about $4700 plus labor. It needs a large gas supply, and a 6" vent through the roof. It will give you about 200 gallons per hour heated to 100º above incoming. I can't tell from your description if you need this unit, or something smaller.