Most systems treat the DHW as a priority zone, and direct all heat to it when needed, so adding extra for that purpose is somewhat counterproductive unless it is something like a commercial spa or someplace where continuous hot water for showers, etc. is required. A typical WH has in the order of 30-50Kbtu. Heat to the house while it is heating the DHW tank has never been an issue, at least for my system - it just doesn't need to be on that long dedicated to that priority zone.
An 80K with modulation would put it in the high efficiency mode most of the heating season and, depending on the size of the WH tank you choose, likely provide all of the hot water you need. Extra for the sake of extra is a waste in both initial cost and running costs. The house doesn't automatically become an icecube, plus, except in rare places, does the house sit for days on end at the design load, so max heat is rarely called for. So, instead of keeping it 72, it falls a degree for each point below the design load.