If Manual-J says your peak heat load is 25K, reality is more likely to be 20K. At that point there's little advantage to going with a gas-fired mod-con over running it off an 80%AFUE forced-draft gas-fired hot-water heater combi like the
Bradford-White Combi2
You'll end up spending about 5 grand less up front, but you'll use about $100-150 more in fuel. With the $5K delta you can buy a
Cansolair or
SolarSheat thermal air panel and easily recoup that $100-150 (in most US locations) and still have couple grand left over to play with.
High efficiency heating systems are for low-efficiency building envelopes. If your ACH50 numbers are less than 1 and your true design day heat load is as low as you're calculating, there's scant rationale for a mod-con. Even the lowest modulating boilers will have less than a 25% duty cycle on average- to achieve the real AFUE at your load it needs to be down there in the 5KBTU range at lowest modulation. The thermal mass of the combi is sufficient to keep it's efficiency pretty much at ~80% through the heating season despite a low duty cycle- you'd have to add thermal mass to a mod-con to keep it happy, and you'd be only 10-12% more efficient.
But the gas hw/heat combi will be a huge improvement over even the smallest oil burners out there (unless you have buffer tank and smart heat-purging controls on the oil boiler.)