Boiler replacement is best left to qualified HVAC specialists. Getting the right load for you home is important, as well as making sure that you have the correct sized specialty items such as expansion tanks. Venting will also need to be addressed with a condensing boiler.

If you will do any future renovations that should also be taken into account.

If you plan to also heat water you want to look at both current and future water usage, not just "averages" Do you have a fancy new bathroom with multihead showers? Three teenagers?

Look at total installed cost and payback, not just "I want the rebate" . Sometimes being steered by a rebate does not put you in the optimal product for your situation.

What's the input and output in Btu of the old boiler?

51MBtu sounds awfully small for heating and DHW - careful you don't undersize it!

SR