You seem to have posted more info as I was typing...
Design temps for your area are around +10F, with a ~5000 heating degree day heating season, so based on an assumed 600 gallons of fuel use for heating we can put an upper bound on your design condition heat load to get a handle on oversizing factors:
source BTUs= 138000 BTU/gallon
At an efficiency 70%, 1-gallon delivers 138000 x 0.7= 96,600 BTUs to the house.
With 600 gallons and 5000 heating degree days, the house uses 600/5000= 0.12 gallons per HDD. (K-factor as used by oil heating guys is the HDD/gallon ratio, which would be 1/0.12 = 8.3 )
Per degree-hour that's 0.12/24= 0.005 gallons so x 96,600BTU/gallons = 483 BTU per degree-hour.
At +10F, you're looking at 65-10= 55 heating degrees, so times 483 BTU/ degree hour you're at ...
26,564 BTU/hr for a design condition heat load, (a credible number, maybe a bit high for a tight well insulated 1400' rancher at 10F) which is about half the output of the very smallest oil burners. If the combustion efficiency is 60%, your actual heat load is more like 22.7K. If (defying available information to the contrary) it really IS 82% would be ~31K ( but I don't believe that for a minute.)
I expect the beastie-boiler's output is north of 100K, so assuming you're in the 25K range for heat load you're at least at 4X oversizing. Using
NORA's FSA calculator tool selecting "old boiler with tankless coil" adjusted to 100KBU output and 64 gallons/day their heat load estimate is about 20KBTU/hr, and your AFUE is about 49% based on 800 gallons/year usage. That estimate may be low, but if so, it's not by much.
At 50% efficiency and $3.50 oil that's 2x $3.50/138=5.1 cents per 1000 BTU. 15 cent electricity is 15/3.412= 4.4 cents per 1000BTU - electric space heaters are literally cheaper.
With a 20K-25K heat load even the smallest Buderus would be 2-3x oversized. Swapping out the AC for a 2.5 ton multi-speed R410A-refrigerant heat pump & variable speed are handler would have the same order of magnitude upfront cost, and lower operating cost. A 3-head 2-2.5 ton multi-split would be more efficient than a ducted system and significantly lower cost, if that works with your floor plan.
Insulating the basement is a good idea, but it's not free either, and it would take the whole-house heat load. If you go that route you have to protect the foundation sill from rising damp from ground moisture, which means no interior vapor barrier, and any studwall needs to be protected from condensation on the studs. The cheap way to do this is to put 1" of rigid XPS insulation up against the foundation wall (seal the seams with duct-mastic), trapping it tight to the wall with a 2x4 studwall with UNFACED R11 or R13 batts. XPS is sufficiently permeable to water vapor to allow the foundation to dry toward the intetior rather than wicking moisture up to the foundation sill, but has sufficient R value that it's interior surface doesn't stay below the dew point of interior air long enough to create moisture problems from wintertime condensation. (You can probably even get away with 3/4" XPS, but half-inch would be pushing your luck.) Be sure to put a sill gasket under the bottom plate of the studwall to ensure that it doesn't wick ground moisture & rot.) It ends up being a ~R15 "whole wall" (thermal bridging included).
Alternatively, using 2-3" of unfaced EPS (white beaded insulation) held in place with 1x strapping through-screwed into the foundation with tapcons on which to mount the wallboard. That ends up at ~ R8-R12. It can be cheaper & easier than the XPS/studwall approach if you have a source for reclaimed rigid board scavenged from commercial roofing demolition/re-roofing. You may have to hunt a bit, but there are resources out there (craigslist.org seems to have several listing in my area). You can use recycled XPS too up to 2" (R10), but be very careful about using foil-faced goods of any type, as that can lead to sill-rot if there isn't a good capillary break between the foundation sill & concrete.
Whatever else you do (or don't), spraying 2" of closed cell foam to insulate and seal the band-joist and foundation sill to eliminate air leaks there is going to be worthwhile, and sealing it to the top of your new insulation scheme would also be key.
If there isn't currently insulation in the floor of your daughter's room, it's not very expensive to blow cellulose between the floor joists from below,and could make all the difference.