What Jim said. I live in a not-super-insulated 1923 house about that size in Worcester, and the -105 would be nearly 3x oversized for my design condition heat load, and I have a
colder outdoor design temp than you. Get a room-by-room heat load.
Also since it has 4 zones, what it the total amount of radiation (baseboard, radiant floor, whatever) on the SMALLEST zones?
If the minimum-fire output of the boiler isn't small enough you can short-cycle it into low efficiency running it at condensing temps if it can't dump that heat into the room at those temps, cutting into the efficiency of the boiler. With any system you'll have fewer problems and can get higher average efficiency by going with the SMALLEST modulating-condensing boiler that actually meets the design condition heat load.
At 120F fin-tube baseboard puts out about 200BTU/foot, @ 140F it's ~300BTU/ft, @ 180F it's ~600BTU/ft.
The min-input of the Alpine-105 is 21KBTU/hr, so at ~ 120F running in condensing mode it's min output is ~20,000BTU/hr.
If your smallest zone has 40' of baseboard, the balance point would be at 20,000/40= 500BTU/ft, which takes water temps of 165-170F. But at 120F the baseboard is only emitting 40 x 200= 8000 BTU/hr....
...so you have 12000BTU/hour more heat going into the system than the baseboard can deliver into the room. What's gonna happen?
The temperature of the loop rises until the boiler turns off because it's overshot the outdoor reset curve temp, then refires when the temp has dropped sufficiently repeat a gazillion time until the thermostat stops calling for heat. How bad is it?
In most boilers the temperature hysteresis is a fixed number of a handful of degrees- lets call it 5F. In 40 foot baseboard with 3/4" pipe you have about 7lbs of water, double that for the boiler mass and distribution plumbing, for about 15lbs of water as the total thermal mass of the zone. At 12000BTU/hr (3.3 BTU/second) of excess heat, to raise 15lbs of water that 5F degrees only takes (15lbs x 5F)/3.3= 25 seconds, after which the boiler stops, waits, refires repeat. Running it that way throws away a fixed amount of heat with every flue purge & ignition cycle, and wears out the boiler.
The min-mod output of the smaller Alpine 80 is ~15000BTU/hour, so a 40' zone that balances at (15000/40=) 375 BTU/hr. The baseboard at 120F dumping 8000BTU into the zone, so your excess now is only (15000-8000=) 7000 BTU/hr (or 1.9BTU/second). The minimum burn time then is (15x 5)/1.9= 39 seconds- still not great, but substantially better than 25 seconds. But if you bumped the temp to 140F the baseboard would be delivering (300BTU/ft x 40=) 12000BTU/hr, so the excess is only 3000BTU/hr, (0.83BTU/second) and your min-burns are 72 seconds, which is much less of a disaster, whereas the bigger boiler with the 20,000BTU min-output would still be short-cycling like crazy on zone calls.
Unless you can run the boiler at 140F or less with burns of well over a minute (5 or fewer burns per hour would be GREAT), you won't be able to get the condensing efficiency without abusing the boiler, and that's why smaller=better, especially with low-mass heat emitters on a multi-zoned system.
But do the math on your actual zones to figure it out. If you have big old-fashioned radiators you have a lot more water-mass buffering it, and the min-burn times soar, not a problem. If you have radiant slab floors, even better! But if you have even one or two stubby zones shorter than 40' of baseboard you have to think about how you want to go about fixing "the problem". And it IS a problem even for the Alpine-80- a BIG problem for the Alpine 150! Adding mass is one way, adding radiation is another, combining zones is yet another, but ideally you'd be able to set the temperature floor of the curve at ~120F for baseboard, if you want to hit 95% efficiency.
With 3 people and even a 50KBTU/hr boiler behind it you won't need to upsize the tank, if the indirect is controlled as a "priority zone". A 50K boiler is nearly 2x the burner of a typical 40 gallon gas-fired standalone. You'd only need to upsize the tank if you were installing a big soaking-tub or spa or something.