There are several condensing propane boilers on the market at 50mbtu input and matched with a 30-40 gallon indirect, would likely be a perfect match for your radiator heated home. All good hydronic heating systems start with a heat load analysis. Degree days are of marginal use, but fuel bills can help. We use Wrightsoft manual J* in conjunction with blower door test where appropriate. We also measure radiation where competitors insist on over-sizing boilers, since it makes no sense to install more boiler than the radiators can handle. This may help.