If you have a recent gas bill that gives an average outdoor temp (or exact billing dates and a zip code to be able to look up degree-day data) it's fairly straightforward to determine an upper bound on your heat load at your 99% outside design temperature working backward from fuel use per heating degree day, and the 84% nameplate efficiency of the existing boiler.
As long as your house has glass in the windows and doors that closed it's unlikely that your heat load at 0F or even -5F is anywhere near 50,000BTU/hr, and it's very likely to be half-that, and below the minimum-firing rate of a Vitodens 100 or Solo 110. If you think we're going into another ice age and need to be covered for a cold snap of -150F these boilers might make sense, but I'm guessing that's a temp not seen in coastal NH in the past 10,000 years.
The type and size of your heat emitters/radiation, and the number of zones also factor in to what makes sense. There are many mod-con boilers in the 50-60KBTU/hr range that modulate down to under 20K, but you may have a heat load more appropriate to a tank hot water solution like a condensing Vertex or something.
The fuel use numbers will tell the biggest part of the story, so let's have 'em! But measuring up the radiation (per zone, if multi-zoned) will also be an important factor for getting it right.