Do you happen to know the efficiency of your current furnace? Obviously a furnace with 100,000btuh rating and a 80% efficiency is equivalent to a 80,000bthu furnace at 100% (if there was such a thing)
Did your old furnace keep you comfortable? Only you can deciide that. A load calculation is a tool to help with the sizing of new furnaces. It's the best tool available for new construction where there hasn't been a furnace in place as a reference, for both the homeowner and the HVAC company. The best thing you could do is get at least three contractors to evaluate what you have and what you need. Let them each make the calculations rather than you tell them what you need; they are the experts. Unless you plan on installing it yourself more quotes will be a better indication as to where you want to be on both sizing and price.