Oil has been running ~$3.5-4+ in MA for a few years now. In the same period NStar's gas grid gas has been running under $1.25/therm, but it might hit $1.50/therm again before the end of the decade (or even the end of the year.) And if gas did hit $1.50 it would still be half the cost of heating with $4 oil, even if you were using the same crappy antique boiler.
A typical steam boiler in a house this old started out as a coal-burner with little or no controls, later retrofitted with an oil-burner and only slightly better controls. As-operated most are only running 45-60% efficiency. A replacement gas boiler properly sized for the radiation really will run 82% if the system is sized properly for the house, but since most of these antiques are 2-3x oversized it's realistically more like 70-75%. If we generously assumed the oil-burner in the existing system is getting 72% efficiency (NO WAY, but just figgrin' ) and replacement boiler's gas-burner delivered only 72%, at $4/gallon oil you'd be getting ~100,000 BTU heat into the system or $40/MMBTU, whereas with $1.50 gas you'd be getting 72,000 BTU, or ~$20.80/MMBTU. But in fact the antique with the retrofit oil burner isn't doing nearly that well, and is probably costing you ~$60/MMBTU.
Steam piping & radiators don't fall apart very easily- the repair nightmare argument is really not very valid
at ALL! That has the ring of a scare tactic for making the sale, but has very little basis in reality.
A steam heating system is simplicity itself, with very few moving parts, and while it does need to be maintained, deferred maintenance doesn't particularly degrade the system, (only the comfort.) Steam systems do need to be re-tuned occasionally, maybe once every decade or so when the system vents & radiator vents start to stick or clog. It's possible to micro-zone 1-pipe steam using
thermostatic vent valves, to better balance the heat distribution or run some rooms colder than others as-desired, with significant fuel savings. It's not the worst thing in the world to "recommission" a functioning steam system, fix any missing or failing pipe insulation, etc., which would almost certainly cost less than cutting up a century old house for a duct-retrofit. A well tuned steam system is very quiet and quite comfortable, and only slightly piggy on the fuel use. And since it doesn't use any power for heat distribution, it's comparable in operating cost to heating with mid-efficiency gas boilers, and can be cheaper than heating with mid-efficiency gas furnaces (steam has no air handlers or pumps load sucking power off the grid.)
Many 2-pipe systems can be reasonably restructured & fitted as a pumped hot water system, in which case it's likely that you could get the efficiency of condensing gas along with improved comfort, and a right-sized boiler capable of hitting it's AFUE numbers (or better), deliver stabler room temperatures, and use only ~2/3 of the gas that a pretty-good steam boiler would on a fully tuned-up steam system. (This is where BadgerBoilerMN is steering you.)
If you are on one of the municipal power companies (not NStar or National Grid) you electricity is pretty cheap, and you COULD heat & cool the place with
ductless air source heat pumps (mini-splits) for about the same operating cost as condensing gas, without running ducts everywhere, using the steam system only as backup.
All condensing gas furnaces are going to be oversized for your actual heat loads, and having it only serving the first floor only compounds the issue- it'll be efficient enough, but it won't be doing anything for comfort, since it'll be cycling on/off a lot. The ducted air source heat pump would cost a somewhat more to operate than condensing gas and you're more likely to find a right-sized solution, but it will not as efficient as ductless mini-splits.
A good heating solution typically starts with a room-by-room load calculation (Manual-J or similar), and sizing the equipment correctly. But before spending money on a new heating system it's also important to seek out and rectify all of the low-hanging fruit on the building envelope, and in most century-plus homes those are many. It starts with air-sealing, followed by retrofit insulation, rebuilding or replacing any ancient windows and adding low-E storms, etc. And don't neglect foundation air-sealing & insulation- even if you're not heating the basement directly an uninsulated air-leaky basement is a large heat load, and insulating the foundation reduces moisture/mold issues while improving first-floor comfort. All of these retrofits affect the heat load calculations and the system sizing (and cost).
Which is all leading up to the point that increasing the efficiency of the BUILDING increases the comfort levels far more than anything you can get out of the heating system (barring radiant floors, which would be a huge cost-adder as a retrofit.) Accept no HVAC proposals that don't come with room-by-room load calculations based on the building construction and the true 99% and 1% outdoor design temperatures, but running those numbers ahead of time on building before & after any envelope upgrades can be eye-opening. (My house just turned 90 this year, but since I've lived here I've brought the heat load down by ~35-40%, while increasing comfort and decreasing heating/cooling costs, even while adding onto the conditioned space.) In NStar & National Grid territory there is
a great deal of subsidy available for building envelope improvements, even though you're heating with oil.
In many 3000' homes in MA a 3/4-ton mini-split per floor is more than sufficient cooling, but would probably come up shy on the heating end at the 99% outdoor design temp, but is often reasonably sized for the January average temp, in which case it would cut the fuel use of the boiler by more than 3/4, at an installed cost under $10K. With oil as the fuel this is a huge payback, with gas, not so much. But if installing air conditioning (ducted or not) sized for the cooling load it's probably going to be worth the upcharge for making it a heat pump rather than cooling only, whether you swap out the boiler for a mod-con or not.