Last question first. A commercial heater may last longer than a residential one, but if it doesn't they normally have much shorter warrranty periods than a residential model. Second, you might be hard pressed to find a commercial model with a 4" flue, and since the burner size is limited by the flue size, the commercial model would not provide any more hot water than the residential unit with the same btu input. With a 4" flue your maximum burner size will be about 75,000 btu. The reason they advise you not to install it yourself is that you may put the new one in exactly like the old one, but there is no guarantee that it was installed properly.