I'd like a drawing of that last pic!
I suppose you could use a HW tank ... but you would have to find a coil to stick in there ... and then keep it leak-free, in order to copy an indirect boiler for cheap. Sounds like too much work for me though. Indirects are pricey, but I'll tell ya -- you'll never run out of hot water again ... EVER.
As an added benefit, I can use mine as a dump zone. When my boiler starts getting over 240° from the HW coils in my handfired, I manually set the hi-limit of the indirect boiler as high as it will go (150°), and that not only pulls all that extra heat out of the boiler, but stores it for hours & hours inside the tank until I need it. They have really good insulation, those indirect tanks.