I use a 10 plate HX (just like the one HD-Tech linked too in his post) that is in the domestic hot water line AFTER the tank. I don't understand why folks think they need to heat the water "before" the tank. IMO it is a total waste of btu's IF you install the plate. The plate gives you an "on demand" hot water system that frankly.....I have never exhausted. Hot showers, wash clothes, dish washer......seriously, you could stay in the shower all day at hot as you can take it water and never run out. My HWH basically becomes a cold water storage tank. My boiler water pump circulates 24/7.
BTW, yes you will need a mixing/tempering valve to lower the water temp for safety.
