Condensing propane is still substantially more expensive than oil.
Run the fuel-use load numbers as outlined in the blog, and measure up all the baseboard (zone by zone totals, if zoned). Some installers will size the boiler to the total amount of baseboard (which is almost always a mistake) to the tune of 500-600 BTU/hr per foot of baseboard rather than the heating demands of the house. Very often the baseboard was longer than needed for the house even before the house got storm windows (or double-pane replacements), and insulation in the walls and attic.
Typical oversizing factors in Worcester seem to run around 3x-4x (rather than the ASHRAE recommended 1.4x), which has efficiency consequences. With smarter controls much of those efficiency hits can be mitigated, but the smaller the boiler, the easier/better that is.
The smallest of the line Buderus G115WS/3 could (theoretically) heat my 2400' house + 1600' basement to 70F at an outdoor temperature of -80F, a temp not seen in Worcester since the last ice age. The G115WS/4 would have me covered down to -120F or so. As ridiculous as that is there are more G115WS/4s being installed out there than G115WS/3s. To even emit the full output of the G115WS/3 takes about 170 feet of baseboard- with less baseboard than that it will cycle on/off taking a loss in efficiency even when all zones are calling for heat.
Don't upsize the boiler to cover domestic hot water heating with an indirect fired tank either- just give the water heater priority with the zone controller. The recovery time with 85,000 BTU/hr of boiler behind it is already about 1/3 the time it takes for a gas or propane standalone water heater. By the time you've stepped out of the shower, dried off and gotten dressed it's done. The temperature in the house won't drop in any meaningful way when holding off the heat for 5-10 minutes to serve the water heater.