The tankless are ok. have to stay within their GPM rating. If you get one too big, it won't come on when you only use a little flow from a faucet. Too small, and you can't take a shower.
Ayuh,.... Do you have a flow control valve in the DWline,....I have a Weil Mclain boiler that is 24 years old and I may decide to upgrade to a new boiler with burner. Now its my understanding that boilers only have a water capacity for domestic use of about 2-1/2 to 3 gallons of hot water. That is the way my boiler is operating now and we have to wait a few minutes before we get hot water again.
First of all your boiler at 24 years is far from old. And this suggestion is something you should seriously consider. A water tank off on it's own loop is a good idea....and you already have the heat source.For fewer problems ,I much perfer the domestic water tanks trhat are looped off the boiler.If you use a lot of hot water a 50 or 80 gallon tank would not let you down.