The "high output" HE3 baseboard puts out about 350 BTU/hr per running foot at an AWT of 120F (the
specs live here),which is where it needs to be to hit 95% condensing efficiency. So the 18' of HE3 is still only emitting 6300 BTU/hr of the necessary 35,000 BTU/hr per zone.
The KS2004 is putting out something like 1800 BTU/hr @ AWT of 120F (a WAG based on the
specified 2095-2240 BTU/hr at an EWT of 130F.)
Assuming the other 38' of baseboard on the first floor is all standard output baseboard it's putting out another 7600 BTU/hr at condensing temps.
Add it all up and you only have enough radiation on the first floor zone to emit 15,700 BTU/hr in condensing mode on that zone. That's less than
half the minimum fire output of the Bosch 151s, and only about 3/4 of the min-fire output of the EFT 110. The all propane versions of the
Vitoden 100 only throttle back to 31,000 BTU/hr-in, 29,500 BTU/hr out in condensing mode, so you'd have about half the necessary radiation to operate
it in condensing mode. The very smallest Vitocrossal 100 can throttle back to about 8500 BTU/hr in condensing mode, but doesn't appear to be marketed in the US. It's not clear of the propane version can go that low. The Vitocrossal 200's or bigger are HUMONGOUS output commercial boilers.
Even running the boiler at 180F you only have enough radiation to emit 45-50,000 BTU/hr. None of the proposals can actual run anywhere near their max firing rate (except when serving the Boilermate)- they would at best modulate in their mid firing range even at non-condensing termperatures
Don't quite know what to say other than installing any of the above (unless it's actually a Vitocrossal 100, and it can modulate to under 15,000 BTU/hr) would be a mistake, since you don't have enough radiation on even your main zone to run in condensing mode without cycling, and would put ridiculously high constraints on the minimum radiation for the upstairs that it would take to keep from short-cycling the boiler into low efficiency and an early grave. These contractors
all flunk the 5th grade arithmetic test on matching the boiler modulation range to the radiation (let alone the space heating load.)
A better bet than the EFT-110 would be HTP's UFT-080w + existing Boilermate. The UFT-080W has a min-fire output of about 7600 BTU/hr, but can still deliver ~70,000 BTU/hr at high fire at non-condensing temperatures, and twice the BTU output of a typical standalone propane water heater. With as little as 40' of standard baseboard per zone it wouldn't short-cycle at condensing temperatures. So even though you won't need 40' of baseboard to heat the upstairs, the UFT=080W you would install 40' (or 22' of HE3), which is probably at least do-able without relocating doors.
There are other boilers out there that can work too. The
Bosch Greenstar 57 can throttle back to ~12,000 BTU/hr- out in condensing mode, yet still puts out ~50,000 BTU/hr at non-condensing temps at high fire. Marrying that to your Boilermate wouldn't be a disaster, but it would take about 60' of baseboard on the upstairs zone to suppress excessive cycling at condensing temps.
A reasonably tight reasonably insulated 2100' house with an 1100' tight but uninsulated basement would usually have a heat load of 45-50,000 BTU/hr @ -10F outside, 70F inside, maybe even less than 40,000 BTU/hr if the basement walls are insulated to IRC 2015 code min (= R15 continuous insulation). If the existing radiation has been keeping the house warm enough, you don't need anything with a high-fire bigger than the Greenstar 57. But the smaller the min-fire the better, to allow operating at condensing efficiency. There are MANY 50-80K-in boilers out there that can serve up less than the 15-16K of condensing radiation you have on the first floor, but find something that modulates to under 10K if you can, or you'll have to ridiculously oversize the radiation on the second floor to keep the boiler happy & healthy.