Hello: It’s best not to mix heating water with drinking water. In some areas it’s even illegal to do so. A single heater can be used if a heat exchanger is used between heating and drinking water. Efficiency-wise, there are all sorts of ways to do things. If a boiler is used, than a storage tank heated by the boiler is a good way to go for potable water. A mod-con, or modulating, condensing boiler is the wave of the future for efficiency and performance.
If you have heat loss calculations done for the addition, it will guide you as to equipment size. If you go to extremes to make the addition more efficient, you could actually save money by not having/needing as much in the way of heat making and distributing equipment. Look into structural insulated panels; SIPS.
Normal/nominal pipe size is the interior, so 3/4″ is really more like 1″ outside diameter, when talking steel pipe. A gas fireplace will only have a 1/2″ line, and you are talking about supplying a heating source for space, water and maybe a dryer. 1/2″ won’t do that 😕