FWIW: Unless you live in the leakiest uninsulated house in Andover or have more glazed area than code allows, that 90K number is almost certainly 2x reality. (My measured heat load at 0F design temp on a circa 1923 ~2000' house + ~1500' of semi-conditioned basement in Worcester is ~30K, and that's with less than R20 in a significant fraction of cathedralized roof in the attic rooms, and a few known gaps in the wall insulation.) Getting rid of the cast iron beast and using a gas-fired tankless HW heater as a boiler + buffer tank I've been able to trim the output downward and modulate with load somewhat. With all zones calling for heat it doesn't put out more than ~42K (needed to bump up the temp for exit air comfort on a hydro-air zone, or I'd back it off even more), and it keeps up just fine, even cycles at -10F, so I'm pretty confident of the actual heat load. Prior to insulating the basement walls, foundation sill & band joist , pounding some cellulose into the 2x4 cavites where it was easy, and the heat load was closer to 43-45K, based on fuel-use history rated against heating degree-day data.
If you have accurate fuel use data on your existing system you can use that (and the boiler's specs) to get a fairly accurate heat load measurement using the FSA tool downloadable from a link on this page.
OTOH, the smallest oil burners are still likely to be 2x oversized for your actual load too, and if you're multi-zoning it with low-mass emitters such as baseboard, the individual zones will be a tiny fraction of the boiler output and you'll have to buffer it to keep it from short-cycling itself into lower efficiency and higher maintenance. If you can figure out what your actual design day water temperature needs are (I't take a WAG at ~140F, which is the min-temp of the return water for a cast-iron oil boiler) it may make sense to use a buffer-centric approach using a reverse-indirect rather than a standard indirect (Everhot EA-series, TurboMax, or Ergomax), with the boiler slaved to the aquastat on the boiler as it's only zone, with the zones sipping from the reverse-indirect/buffer, like this:
That way there's a minimum burn time determined by the mass of the buffer/indirect- it can't short-cycle, and the water temp or room temp doesn't need a huge hysteresis to keep the boiler from short-cycling. Under long hot water draws the boiler kicks and it'll deliver pretty much a full shower flow continuously, and has the mass of the tank to back it up a bit. If you set the tank's aquastat to about the design day heating water requirement, it'll probably be a reasonable temp for the DHW load as well, but you'll still have to install an thermostatic mixing valve at the DHW output should you ever need/want to crank the tank temp higher.
I've read bloggery in recent years that the fuel availble in the US isn't really up to snuff for running a condensing Buderus- I'd research that a lot closer before going that route. Their non-condensing models have a good rep.
Given that oil demand worldwide looks like demand will continue to increase faster than supply for at least a decade prices on heating oil are likely to continue to be very volatile. With the Allegheny shale gas now coming on line, the future for natural gas in New England are likely to be both more stable and continue to be much lower per BTU- if you're anywhere near the local gas grid, this might be the time to switch fuels, and you'd have quite an array of modulating/condensing options (in which case you might NOT want to use a buffer-centric system architecture.)
Last, not least, if it's not too late, DO insulate your basement walls before finishing it out, and do it in such a way that you don't create a mold/rot problem. See:
I did mine at ~R20 using recycled 3" semi-permeable rigid iso, but 2-3" of EPS and unfaced batts in a studwall, no interior vapor barrier gets you there too. Recycled rigid board goods can be had on the cheap locally in Framingham at The Insulation Depot. Unless you know for certain that you have a capillary break at the foundation sill, don't use any foil-faced rigid-board, and if you use XPS (pink/blue) you can't go more than 2" before it cuts the drying capacity of the foundation too much. You can put up to 4" (~R16) of un-faced EPS though. With an all-foam-board approach you can hold it in place with furring through-screwed into the foundation with tap-cons and hang the gypsum on the furring. If you have a fieldstone foundation 2" of closed cell foam + unfaced batts in a studwall are probably the better approach. Set the studall up first, and make sure the foam seals up to the stud edges before adding the batts. Put 6mil poly under the floor plate of the studwall as a capillary break.