Great project. I'm now old enough to remember how cutting edge and economic a combination oil/wood furnace was. I remember cutting splitting stacking drying stacking again wood for the furnace. It seemed to good to be true having the house so hot you would open the windows in a snow storm and wake up to a cold house having to build another fire. OOH the oil we saved.
Anyway The little down-side may be the constant air-flow generated by the chimney and in thru the oil burner and fire door. As you warm the water jacket the cooler basement air will flow in thru these things cooling your water and warming the chimney. Are you sure you want to place these beautifuly created highly efficient HPs onto a leaky old dinosaur instead of a nice well insulated electric hot water tank with one element removed on the bottom for the extra inlet you need for the HX. The top element you can leave for a little temp boost or even a dump for any extra solar power you may have. I think even a good used tank could be found. Just add one of those insulation kits from the Home Depot and presto another beautiful highly efficient envy of the eco-renovators ASHW tank.