It is not uncommon for older units to be 3-4x oversized. Do not use the x BTU/sq ft as it is not very accurate. Depending on how the domestic hot water was heated (internal coil vs indirect tank) would somewhat determine what you need for your hot water. 43K may in fact still be high. There are several threads on-going that discuss this, but one way to determine what your actual heating needs are would be to use your gas bill. The amount of energy you use run against the environmental information (how many heating degree days and similar info) can give you a very accurate indication of your exact situation, rather than a guess, as it takes into account the real insulation, windows, infiltration, etc. losses in your house. Anything else is an approximation. Some of them are really conservative.
Keep in mind that the highest level of comfort results in the water temp used for heating to be adjusted and the boiler running 100% of the time. A mod-con can do this, a single burn rate boiler can't. Your old system probably short cycled itself to death.
Whether a mod-con would pay for itself over another technology depends on the cost of fuel. I doubt it would pay for itself in 5-years, but may make the place easier to sell.