I have a Boush tankless in my house,just the two of us now so I didn't go for the bigger one..I change around 700 ish gal per day and my heater will put out the 84 deg water all day long.Yea it cost more than a water tank..Mine was in the 1200 $ range installed,but the pleasure of changing all the water I want whenever I want is well worth the cost.
I would say do some research first and call a good plumbing company to come out and look at what you need and follow their recommendations
Good luck
Mench