Getting a reasonable heat load calculation is the first and more critical part to getting it right. Anybody quoting on it WITHOUT running the heat load calc first is pretty much a hack, to be avoided.
When running a heat load calculation, don't put any thumbs on the scale such as unrealistically low outdoor design temps or high indoor temps. If if you like to keep it to 75F and it's been -10F at least once in Stamford since the last ice age, use 70-72F at most, and the 99% outside design temp (which for Stamford CT is about 14-15F.)
Let's take a rough-cut first stab at it, which is probably close enough to question any body else's calc that comes in higher:
Assuming the 1950s construction was 2x4s 16" o.c. with R11 rock-wool or fiberglass the U-factor on the walls is about 0.11 BTU per square foot per degree-F after the thermal bridging of the framing is factored in. Assuming +15F & 70F as the outdoor/indoor design temps, that's a 55F delta, so your wall losses are about:
0.11 x 55F x 1520'= 9196 BTU/hr
Assuming you have clear glass (not low-E) storm windows over wood sash double hungs the window U-factor is about 0.50 and the window losses are about:
0.50 x 55F x 225' = 6188 BTU/hr.
Lets' assume you have at least R19 in the ~ 608' of attic on 2x6 16"o.c. joists, which is a U-factor of about 0.075 after thermal bridging, so the ceiling losses are about:
U0.075 x 55F x 608' = 2508 BTU/hr
Assuming an uninsulated poured concrete foundation you have about a U-1 on the above-grade portion of the foundation. You have a perimeter of about 100', and assuming 4' of above ground exposure (counting down to a foot below grade to account for the ground losses) that would be ~400 square feet. And when it's +15F outside it's probably only 60F in the basement so your looking at a 45F not a 55F delta, making your basement losses:
U1 x 45F x 400= 4500 BTU/hr.
Figure another ~45' of U-0.5 exterior door for another:
U0.5 x 55F x 45'= 1238 BTU/hr.
Add it all up you get 23,630 BTU/hr.
If the place is pretty drafty add a 25% fudge-factor for infiltration and you're at (1.25 x 23,630=) 29,537 BTU/hr, call it 30 KBTU/hr
If the place is pretty tight it's probably more like 25K.
A modulating condensing boiler that has any more than 15K of output at minimum-fire is a model to be avoided, since even at your AVERAGE mid-winter outdoor temp it would be cycling rather than modulating. (There's simply no-point to a modulating boiler unless it's modulating MOST of the time in winter!)
The Baxi Luna Duo Tec series Model 40GA has a min-fire of 18K- a bit more than you really want, but maybe.
The TT Prestige Excellence has an even higher min-fire of 30K which makes it even worse- forget about it. (The Prestige Trimax Solo 60 dials back to 16K-in 15K out, which is about right.)
The Navien CH-180-ASME-NG (smallest of the series) pulls down to 17K in/16K out, which is probably going to be OK. The bigger versions have more hot water heating output, but also a higher min-fire. This is the one, if you go with that series.
But if you really have that much uninsulated basement it may be better to go with something cheaper a tiny forced draft cast iron boiler or a combi-tank HW heater and spend the difference in upfront cash on insulating the foundation walls and air sealing the whole house, which would likely bring your load to under 20K. When heat loads are under 20K there's no way you can get much modulating, and depending on the fin size on those convectors it's likely that you can do just fine even at +10F outdoor temps with 140F water, making a condensing tank-type water heater solution pretty reasonable.
So, what are the depth & height of the fins on those fin-tube convectors?