Out with the oil and in with the gas. Which boiler?

My dad has proposals from three contractors for oil-fired boiler conversion to gas. He is not seriously considering two offers from these contractors to replace the oil burner with a gas burner and leave the 55 YO Crane Sunnyday in place. That old soldier, in case you are not familiar, provides heat and DHW (with a coil). He is alone in the house most of the time with weekend and vacation time family visitors. He is in central NY. (There are lots of heating degree days there.) There are fin-tube radiators in three zones, one very small.

Two contractors proposed a Weil-Mcclain GV90+ Neither of these included an outdoor control. They are probably correct in their conclusion that dad will not live long enough to pay back the price in energy savings for that option. Are there other advantages to that kind of control that should be considered?

One with a PurPro 40 indirect water heater (Pro41Z140)
The other one with a Bradford White PowerStor 30 gallon indirect.

One of the above contractors also proposed a Baxi DuoTec (BAXLUNADUOTEC40KIT). Again, no outdoor reset.

A third contractor proposed a Rinnai E110C. Apparently, this comes with a outdoor reset as standard equipment.

I know that the combination boilers will not provide hot water as fast as the indirect heaters. I hope that they will match what he has now. There are some price differences, and there are differences in what will be done with the existing oil in the tank, buy back or give away.

What I’d like are some comments on the operational differences in these boilers. (I see that the Baxi is a 7:1 whereas the Rinnai is 5:1.) In addition to that, any comments about reliability would be nice and any input about installation complications. I am having a hard time comparing the DHW output of the modulating boilers. The Baxi literature says that will make 3.9 gpm but give not temp differential to go with that. The Rinnai says 3.2 gpm at 75F delta.

There are things that I do not understand about the boilers. How are the Rinnai and Baxi able to handle a constant flow-through of oxygenated water while the Weil Mcclain is not? Is there a minimum water flow to activate water heating with these combination boilers? Does space heating output drop when DHW is made? The Rinnai installer says that he can go out the side of the house with flue gas, but the others say that they need to put pipe up the existing chimney due to window locations. Is there a difference with the Rinnai that dictates this?

Thanks for reading