The R value of the framing is a slightly optimisitic in that Colorado Energy calc as are the contributions of air films, but it's "good 'nuff."
Whatever you calculated for floor losses drops by something like 75% if you insulate the foundation walls to R10 or better (R15 continuous insulation is IRC 2012 code min for US climate zone 5, which includes CT). So if the calc came in at ~15,000 BTU/hr, it'll drop to something like 3000-4000 BTU/hr. If you insulate the slab too it drops even further, but that's a separate discussion. (Probably not worth doing unless you can tolerate the hit in ceiling height.
Even if you don't insulate any other part of the basement on the first go-around, if mounting the boiler on an exterior wall, put at least R10 between the boiler & plumbing manifolds when you install it, since it would be a super PITA to deal with that later. Putting 1.5-2" of foam (any type) against the wall and building a studwall with unfaced batt insulation to the interior side of that with 3/4" OSB or plywood facing the interior gives you something solid to mount the boiler, pumps & plumbing onto. Put an inch of EPS or XPS under the bottom plate of the studwall as a capillary & thermal break to keep the bottom plate from accumulating moisture through the slab (or summertime air.) TapCon the bottom plate to to the slab through the foam.
Upsizing the boiler for domestic hot water is insane if you're heating water with an indirect running as "priority" zone. The output of a mod-con is higher than most 40-50 gallon standalone tanks. You might have issues with the smaller boiler if you're heating an outdoor hot tub, but unless there are special circumstances like that, size the indirect for the biggest tub you have to fill, and size the boiler for the space heating load.
When I started upgrading my place about a decade ago ( a 2400' circa 1923 1.5 story bungalow in Worcester MA, 99% outside design temp +5F) I ended up scrapping a Burnham P206 (DOE output of about 130K, IBR of about 120K) that was only about 12-15 years old. The heat load characteristics of some of the rooms were SO different from the main areas that I decided to micro zone, and centered the heating system around a ~50 gallon buffer tank (that is also a "reverse indirect", with a potable coil inside the tank), and designed the radiation to deliver the heat at a single temp 125F AWT. When all zones are calling for heat the entering water temps at the boiler are around 115, with the boiler output of ~145F, and a buffer tank set point of 130F.
Manual-J calcs put my design load in the mid-30s, IBR methods put it at ~40K, so it's very similar calculated loads to yours. Note the pre-existing boiler was more than 3x oversized(!) and was more than 2x oversized for the load even before air sealing & insulating. I'm radiation limited to something between 40-45K of output, at the water temps I'm running, and the place sails through -5F & cooler weather just fine. The exterior surface area, insulation values, and window U-factors of your house are all better than mine EXCEPT for the foundation insulation. You really don't need more than 40K of boiler even before insulating the foundation, and would have significant temperature margin below +7F outdoor temps with a 40K boiler after insulating the foundation, shedding another 10K or more of peak load.
My radiation is a kludgey combination of radiant floors, re-used re-painted antique Arco SunRad recessed radiators (architecturally appropriate for a 1920s house), a section of cast iron baseboard (for the upstairs bathroom), with the "Hail Mary" backup being the pre-existing hydro-air air handler that had been served by the oversized P206. The air handler is capable of heating the whole house on it's own even with 120F water, since it too was 3x oversized, but the distribution & temperature balance between rooms is pretty lousy. When it's under 10F outside (or if using deep setbacks) the hydro-air kicks on, but most of the time the radiation is carrying the load. With outdoor reset I'd be able to heat it totally with the radiation 100% of the time, but I wouldn't run the buffer setput cooler than 125F under any circumstances, for domestic hot water performance reasons. Setting up the boiler for a fixed output of 140-150F the entering water is always in the condensing zone (with a reasonable delta-T) except for the last minute or so of burns when NOTHING is calling for heat, and it can't short-cycle on zone calls even though the smallest zone can only emit ~2500BTU/hr, (well under the minimum modulation level of any mod-con.)
Had the HTP Versa been available at the time it probably would have been a better (and cheaper) choice, and easier to design in, but technology marches on, eh?