Hello: I’d agree that a boiler and indirect are going to be overkill for domestic hot water, unless you have a rather large family. With that kind of heat outside, there should be a way to use solar. I do like heat recovery as it has very little to go wrong. You may want to look at the Phoenix Solar by HTP. Basically it has a mod-con boiler installed halfway up in a stainless tank. At the bottom is a coil that gets hooked up to solar to do the bulk of the heating. Doing both DHW and space with this solar/mod-con appliance could be a good thing 😎
I suspect that as ambient temp goes up, you’re going to get less condensation of flue gasses… but if it’s also hooked up to solar, the boiler isn’t going to be firing then anyway. The bigger problem to me is if the water being heated is too hot to allow condensation. Particularly if space heating is involved, keeping distribution piping temperature low is a worthy goal.