Using the NORA FSA calculator and putting in an 83% boiler w/tankless coil with 96K of output, 2% idle loss, using Hartford's weather data, 990-1000 gallons/year gives you an estimated whole house heat load of ~38K at a design temp of -1F. If we back off the steady-state efficiency estimate to 80% it's giving us ~36K @-1F. Backing off to 78% gives us ~35K. No matter what your real-actual design day heat load is no more than 40K. For analysis purposes lets use 36K. Heat loss calculators typically hit 15-25% to the high side, so don't through the contractor out on his ear if he's saying 45K, but if they come back with 60K they've made significant errors or very wrong assumptions.
The average as-used efficiency due to oversizing for the 83% steady state efficiency case is 68%.
The average as-used efficiency due to oversizing for the 80% steady state efficiency case is 65%.
Modeling the Viessmann + indirect at 90K output, 0.3% standby idle-loss the as-used AFUE up to ~80% (after adjusting the fuel use numbers down to where it indicates a ~36K load with that boiler which, is at ~800 gallons/year.)
Modeling the Buderus G115WS-3 w/85K of output, same standby ends up with similar results- ~800 gallons/year, ~80% as-used AFUE.
Unless you have a big tub to fill there's little to be gained by going with a bigger indirect- the burner output alone is enough to support an endless 2gpm shower flow, the mass is only buffering for the intermittent peaks of multiple draws. If you have a big soaking-tub or spa to fill the bigger indirect lets you fill it somewhat faster, but that's about it. When operated as a separate zone the mass of the tank does nothing for your space-heating efficiency.
The water temp required to deliver 36K to the house depends on the total running length of baseboard. To be able to run with 140F water at design conditions would take between 110 -120 feet of fin-tube. What ultimately determines the design condition water temp is the zone-by-zone heat-loss/baseboard length ratio though, which could be higher than the whole-house heat-loss/baseboard ratio. But measure it- the optimal outdoor reset curve depends on it, and it may affect how the system is plumbed to the boiler.
DO ask the contractors about the short-cycling on zone at low water temps using outdoor reset, and make them calculate a minimum burn time at the lowest operating output temp that the boiler tolerates. If it's under 5 minutes (which it almost surely is) ask about the cost to add a buffer. (Print out the system diagram pictures as well.)
FWIW: As a quick exercise, while propane is more expensive in terms of source-fuel BTUs, a right-sized modulating condensing propane boiler is possible, and would deliver north of 90% efficiency, which could make it cheaper to operate. There are MANY propane fired mod-cons with minimum modulated output under 20K, and the max output of even the smallest are typically over 45K.) Something like a Triangle Tube Solo 60, Peerless Pinnacle T-50, or Burnham Alpine ALP080 would be a perfect fit, and would deliver long modulated burns at high efficiency (fewer and higher efficiency if the system is buffered with some mass), and you'd be able to pull a realistic 92-93% as-used AFUE out of it. In source BTUs/gallon terms you get ~138KBTU out of a gallon of oil, 92KBTU out of a gallon of propane. So at your projected new-boiler 80% AFUE you're averaging 110KBTU/gallon of heat delivered to the house (or hot water), or with a 92% AFUE propane burner, ~86KBTU/gallon. So you'd get 28% more delivered-BTUs/gallon with oil than with propane, but is the price per gallon 28% more than propane? (Usually not, in MA anyway.)