My opinion FWIW is that the a seperate hotwater tank, like a boilermate, won't save you much if anything over the boiler installed coil. The reason is that the Harman boiler will be running 24/7 albeit in pilot mode, and will be idling with a water temp of about 140 to 150 degrees or so which is fine for making hot water. It won't matter if the coil is installed in the boiler or if you have a seperate tank, the boiler will still be loosing heat to the surroundings, and burning the same amount of coal. The VF3K has about 50 gallons of water so reserve capacity is very good also.
If it were a gas or oil fired system, where the burner came on just long enough to supply the heat demand, then yes the tank is the way to go.
You can buy the boiler coil for around $200 or so or spend upwards of $1000 for the tank with coil.