It's almost never cost effective to install two boilers when one can do the job. With a 10:1 turn down ratio an
HTP UFT-120W (or the
Westinghouse labeled version of the exact same boiler, distributed through Home Depot in some states) can vary it's output from about 11,400 BTU/hr to 114,000 BTU/hr. Even at 120F it only takes 65' of baseboard to emit the full 11.4K. A
Lochinvar WHN111 would be pretty similar, modulating between 10,500 - 105,000 BTU/hr.
But it probably doesn't even need to be that big. I suspect the fuel use load calc on the house as-is/as-was prior to improvements is going to come in at about 80K, and after insulating the basement and doing more air sealing it'll be in the 70K range, which would make an 80-85K input fire tube boiler with a 10:1 turn-down ratio a reasonable choice, 100-105K max.
It's not impossible to install a UFT-xx boiler as a DIY, but study up on hydronic systems (a LOT) before diving in. That series comes pre-plumbed with a separate port to support an indirect water heater, but with the shiny new Rinnai already in place there's no reason to go there. Understanding the min & max flow requirements for both the boiler and system radiation is important to making this work. It's way more than just a plumbing exercise, and it's probably worth have a hydronic design professional spec the system, including all the pumps, valves, etc even if you do the work.
In your boiler list you're using net water output numbers where you SHOULD be using the DOE output numbers, since it's all inside of conditioned space. eg: The Weil Mclain ECO 110 has a DOE output of 101,000 BTU/hr, not 88,000 BTU/hr.
The ECO 110 is a nice boiler, but with only a 5:1 turn-down ratio, which means it takes about
twice the amount of radiation the similarly sized WHN 111 or UFT 120W to keep it from short cycling on the basement zone. Minimum firing rate is as important as maximum when
sizing a modulating condensing boiler, especially when there are low-load or minimal radiation zones to deal with.