Indirect hot water heater is the best solution in my opinion. Even if you get the old heat exchanger out you need to buy a new heat exchanger that will thread in to your boiler. By using a Indirect hot water heater you can put the hot water tank near the hot water demand.
Just remembered the only problem with Indirect hot water heaters is some models don't let you drain the fresh water out of them and that makes them hard to protect in freezing conditions, most people don't worry about this but I drain my house when I go on vacation. I live in Alaska, may not be a concern to you.