The first thing is, without a good manual-j heat load analysis, you're shooting in the dark. Going by the info you gave is not enough to get a WAG. Now, you can get a WAG, or better than that, by using the oil used/verses heating degree-day info, but it is only an approximation (usually, upper bound). A condensing boiler may or may not be a good choice. You have a misconception about the low temps...with an outdoor reset and a properly sized and installed condensing boiler, you only need higher temps on a really cold day, and the thing is likely to be able to condense a good portion of the time. The hassle is with four zones, if only one is calling for heat, your heat load is likely to be below the minimum of even a modulating boiler which affects any boiler's efficiency. Adding an indirect WH as a zone does NOT constitute a good reason to make a boiler larger. Size it properly, too for the anticipated load, and you should never run out of hot water or notice any degradation of comfort in the house.