Looking to do chimney-vent with buffer tank and outdoor reset

I'm assuming you need a buffer tank because the boiler is oversized and you want to make sure the burner runs long enough to make sure you don't have any condensing in the chimney or boiler.

Having an indirect water heater with the buffer tank is where it gets interesting. The indirect needs to get heated to 140⁰ ideally and the buffer tank is probably going to need to be heated around 140⁰ as well to avoid condensation. Why not combine them together? Get an indirect tank with a big coil that has a lot of volume and pipe it as a reverse indirect. You can use a Taco i series outdoor reset capable mixing valve or use a variable speed injection pumping primary/secondary piping strategy. I'm sure Caleffi Idronics has examples of how to do it.